[X86] Fix a warning

This patch fixes:

  llvm/lib/Target/X86/X86ISelLowering.cpp:47257:15: error: comparison
  of integers of different signs: 'int' and 'size_t' (aka 'unsigned
  long') [-Werror,-Wsign-compare]

Author: kazutakahirata

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -47254,7 +47254,7 @@ static SDValue combineSelect(SDNode *N, SelectionDAG &DAG,
       for (auto [I, M] : enumerate(ByteMask)) {
         // getConstVector sets negative shuffle mask values as undef, so ensure
         // we hardcode SM_SentinelZero values to zero (0x80).
-        if (M < ByteMask.size()) {
+        if (M < (int)ByteMask.size()) {
           LHSMask[I] = isUndefOrZero(LHSMask[I]) ? 0x80 : LHSMask[I];
           RHSMask[I] = 0x80;
         } else {