LLVM disassembler can not decode POPCNT with REX2 prefix correctly

[KanRobert]

bash$ cat 1.s
popcntl %r16d, %r17d

bash$ llvm-mc --show-encoding 1.s
.text
popcntl %r16d, %r17d # encoding: [0xf3,0xd5,0xd0,0xb8,0xc8]

bash$ cat 1.txt
0xf3,0xd5,0xd0,0xb8,0xc8

bash$ llvm-mc --disassemble 1.txt
.text
rep popcntl %r16d, %r17d

Expect no rep prefix b/c 0xf3 is part of opcode here.

[llvmbot]

@llvm/issue-subscribers-backend-x86

Author: Shengchen Kan (KanRobert)

bash$ cat 1.s popcntl %r16d, %r17d

bash$ llvm-mc --show-encoding 1.s
.text
popcntl %r16d, %r17d # encoding: [0xf3,0xd5,0xd0,0xb8,0xc8]

bash$ cat 1.txt
0xf3,0xd5,0xd0,0xb8,0xc8

bash$ llvm-mc --disassemble 1.txt
.text
rep popcntl %r16d, %r17d

Expect no rep prefix b/c 0xf3 is part of opcode here.