[llvm] [X86] Fix an assembler bug of CMPCCXADD. (PR #66748)

[Shengchen Kan via llvm-commits]

================
@@ -768,3 +768,6 @@
 // CHECK: encoding: [0xc4,0x62,0xa9,0xe4,0x8a,0x00,0xfc,0xff,0xff]
----------------
KanRobert wrote:

Why was it encoded correctly before this PR? I think r9 needs R bit too.

https://github.com/llvm/llvm-project/pull/66748