compress: Elide long-offset path on 64-bit

ZSTD_seqToCodes() only enables longOffsets on 32-bit builds because the
64-bit bitstream accumulator can hold every supported offset. However,
ZSTD_encodeSequences_body() still tests the runtime flag for the first
sequence and every sequence in its hot loop.

Make the architecture condition explicit at both branches so 64-bit
compilers can discard the unreachable path. Add an assertion documenting
the invariant while preserving the existing 32-bit behavior.

In five alternating GCC 11.5 fullbench runs encoding sequences for an
8 MiB, 50%-compressible level-3 sample, throughput increased from
1201.6 to 1213.4 MB/s on average (+1.0%) with identical 2,706,887-byte
output. The fullbench text section also shrank by 512 bytes.

Signed-off-by: Usama Arif <usama.arif@linux.dev>

Author: Usama Arif

lib/compress/zstd_compress_sequences.c

@@ -300,6 +300,7 @@ ZSTD_encodeSequences_body(
     FSE_CState_t  stateOffsetBits;
     FSE_CState_t  stateLitLength;
 
+    assert(MEM_32bits() || !longOffsets);
     RETURN_ERROR_IF(
         ERR_isError(BIT_initCStream(&blockStream, dst, dstCapacity)),
         dstSize_tooSmall, "not enough space remaining");
@@ -315,7 +316,7 @@ ZSTD_encodeSequences_body(
     if (MEM_32bits()) BIT_flushBits(&blockStream);
     BIT_addBits(&blockStream, sequences[nbSeq-1].mlBase, ML_bits[mlCodeTable[nbSeq-1]]);
     if (MEM_32bits()) BIT_flushBits(&blockStream);
-    if (longOffsets) {
+    if (MEM_32bits() && longOffsets) {
         U32 const ofBits = ofCodeTable[nbSeq-1];
         unsigned const extraBits = ofBits - MIN(ofBits, STREAM_ACCUMULATOR_MIN-1);
         if (extraBits) {
@@ -353,7 +354,7 @@ ZSTD_encodeSequences_body(
             if (MEM_32bits() && ((llBits+mlBits)>24)) BIT_flushBits(&blockStream);
             BIT_addBits(&blockStream, sequences[n].mlBase, mlBits);
             if (MEM_32bits() || (ofBits+mlBits+llBits > 56)) BIT_flushBits(&blockStream);
-            if (longOffsets) {
+            if (MEM_32bits() && longOffsets) {
                 unsigned const extraBits = ofBits - MIN(ofBits, STREAM_ACCUMULATOR_MIN-1);
                 if (extraBits) {
                     BIT_addBits(&blockStream, sequences[n].offBase, extraBits);