[huf] Improve fast C & ASM performance on small data

* Rename `ilimit` to `ilowest` and set it equal to `src` instead of
  `src + 6 + 8`. This is safe because the fast decoding loops guarantee
  to never read below `ilowest` already. This allows the fast decoder to
  run for at least two more iterations, because it consumes at most 7
  bytes per iteration.
* Continue the fast loop all the way until the number of safe iterations
 is 0. Initially, I thought that when it got towards the end, the
 computation of how many iterations of safe might become expensive. But
 it ends up being slower to have to decode each of the 4 streams
 individually, which makes sense.

This drastically speeds up the Huffman decoder on the `github` dataset
for the issue raised in #3762, measured with `zstd -b1e1r github/`.

| Decoder  | Speed before | Speed after |
|----------|--------------|-------------|
| Fallback | 477 MB/s     | 477 MB/s    |
| Fast C   | 384 MB/s     | 492 MB/s    |
| Assembly | 385 MB/s     | 501 MB/s    |

We can also look at the speed delta for different block sizes of silesia
using `zstd -b1e1r silesia.tar -B#`.

| Decoder  | -B1K ∆ | -B2K ∆ | -B4K ∆ | -B8K ∆ | -B16K ∆ | -B32K ∆ | -B64K ∆ | -B128K ∆ |
|----------|--------|--------|--------|--------|---------|---------|---------|----------|
| Fast C   | +11.2% | +8.2%  | +6.1%  | +4.4%  | +2.7%   | +1.5%   | +0.6%   | +0.2%    |
| Assembly | +12.5% | +9.0%  | +6.2%  | +3.6%  | +1.5%   | +0.7%   | +0.2%   | +0.03%   |

lib/decompress/huf_decompress_amd64.S

@@ -131,7 +131,7 @@ HUF_decompress4X1_usingDTable_internal_fast_asm_loop:
     movq 88(%rax), %bits3
     movq 96(%rax), %dtable
     push %rax      /* argument */
-    push 104(%rax) /* ilimit */
+    push 104(%rax) /* ilowest */
     push 112(%rax) /* oend */
     push %olimit   /* olimit space */
 
@@ -156,11 +156,11 @@ HUF_decompress4X1_usingDTable_internal_fast_asm_loop:
     shrq $2, %r15
 
     movq %ip0,     %rax /* rax = ip0 */
-    movq 40(%rsp), %rdx /* rdx = ilimit */
-    subq %rdx,     %rax /* rax = ip0 - ilimit */
-    movq %rax,     %rbx /* rbx = ip0 - ilimit */
+    movq 40(%rsp), %rdx /* rdx = ilowest */
+    subq %rdx,     %rax /* rax = ip0 - ilowest */
+    movq %rax,     %rbx /* rbx = ip0 - ilowest */
 
-    /* rdx = (ip0 - ilimit) / 7 */
+    /* rdx = (ip0 - ilowest) / 7 */
     movabsq $2635249153387078803, %rdx
     mulq %rdx
     subq %rdx, %rbx
@@ -183,9 +183,8 @@ HUF_decompress4X1_usingDTable_internal_fast_asm_loop:
 
     /* If (op3 + 20 > olimit) */
     movq %op3, %rax    /* rax = op3 */
-    addq $20,  %rax    /* rax = op3 + 20 */
-    cmpq %rax, %olimit /* op3 + 20 > olimit */
-    jb .L_4X1_exit
+    cmpq %rax, %olimit /* op3 == olimit */
+    je .L_4X1_exit
 
     /* If (ip1 < ip0) go to exit */
     cmpq %ip0, %ip1
@@ -316,7 +315,7 @@ HUF_decompress4X1_usingDTable_internal_fast_asm_loop:
     /* Restore stack (oend & olimit) */
     pop %rax /* olimit */
     pop %rax /* oend */
-    pop %rax /* ilimit */
+    pop %rax /* ilowest */
     pop %rax /* arg */
 
     /* Save ip / op / bits */
@@ -387,7 +386,7 @@ HUF_decompress4X2_usingDTable_internal_fast_asm_loop:
     movq 96(%rax), %dtable
     push %rax      /* argument */
     push %rax      /* olimit */
-    push 104(%rax) /* ilimit */
+    push 104(%rax) /* ilowest */
 
     movq 112(%rax), %rax
     push %rax /* oend3 */
@@ -414,9 +413,9 @@ HUF_decompress4X2_usingDTable_internal_fast_asm_loop:
 
     /* We can consume up to 7 input bytes each iteration. */
     movq %ip0,     %rax  /* rax = ip0 */
-    movq 40(%rsp), %rdx  /* rdx = ilimit */
-    subq %rdx,     %rax  /* rax = ip0 - ilimit */
-    movq %rax,    %r15   /* r15 = ip0 - ilimit */
+    movq 40(%rsp), %rdx  /* rdx = ilowest */
+    subq %rdx,     %rax  /* rax = ip0 - ilowest */
+    movq %rax,    %r15   /* r15 = ip0 - ilowest */
 
     /* rdx = rax / 7 */
     movabsq $2635249153387078803, %rdx
@@ -426,7 +425,7 @@ HUF_decompress4X2_usingDTable_internal_fast_asm_loop:
     addq %r15, %rdx
     shrq $2, %rdx
 
-    /* r15 = (ip0 - ilimit) / 7 */
+    /* r15 = (ip0 - ilowest) / 7 */
     movq %rdx, %r15
 
     /* r15 = min(r15, min(oend0 - op0, oend1 - op1, oend2 - op2, oend3 - op3) / 10) */
@@ -467,9 +466,8 @@ HUF_decompress4X2_usingDTable_internal_fast_asm_loop:
 
     /* If (op3 + 10 > olimit) */
     movq %op3, %rax    /* rax = op3 */
-    addq $10,  %rax    /* rax = op3 + 10 */
-    cmpq %rax, %olimit /* op3 + 10 > olimit */
-    jb .L_4X2_exit
+    cmpq %rax, %olimit /* op3 == olimit */
+    je .L_4X2_exit
 
     /* If (ip1 < ip0) go to exit */
     cmpq %ip0, %ip1
@@ -537,7 +535,7 @@ HUF_decompress4X2_usingDTable_internal_fast_asm_loop:
     pop %rax /* oend1 */
     pop %rax /* oend2 */
     pop %rax /* oend3 */
-    pop %rax /* ilimit */
+    pop %rax /* ilowest */
     pop %rax /* olimit */
     pop %rax /* arg */