[X86][MC] Not decode 0xf3 as rep prefix if it's right before REX2

This fixes #95412

Author: KanRobert

llvm/lib/Target/X86/Disassembler/X86Disassembler.cpp

@@ -284,7 +284,10 @@ static int readPrefixes(struct InternalInstruction *insn) {
       //      it's not mandatory prefix
       //  3. if (nextByte >= 0x40 && nextByte <= 0x4f) it's REX and we need
       //     0x0f exactly after it to be mandatory prefix
-      if (isREX(insn, nextByte) || nextByte == 0x0f || nextByte == 0x66)
+      //  4. if (nextByte == 0xd5) it's REX2 and we need
+      //     0x0f exactly after it to be mandatory prefix
+      if (isREX(insn, nextByte) || isREX2(insn, nextByte) || nextByte == 0x0f ||
+          nextByte == 0x66)
         // The last of 0xf2 /0xf3 is mandatory prefix
         insn->mandatoryPrefix = byte;
       insn->repeatPrefix = byte;

llvm/test/MC/Disassembler/X86/apx/rex2-format.txt

@@ -21,10 +21,10 @@
 # INTEL: movsxd	r17, r16d
 0xd5,0x58,0x63,0xc8
 
-# ATT: rep
-# ATT-SAME:   popcntl %r16d, %r17d
-# INTEL: rep
-# INTEL-SAME: popcnt r17d, r16d
+# ATT-NOT: rep
+# ATT:   popcntl %r16d, %r17d
+# INTEL-NOT: rep
+# INTEL: popcnt r17d, r16d
 0xf3,0xd5,0xd0,0xb8,0xc8
 
 ## MRMSrcRegCC