[llvm] [X86] Ignore REX prefixes not immediately before opcode (PR #117299)

[Ondřej Sýkora via llvm-commits]

================
@@ -770,3 +770,8 @@
 
 # CHECK: prefetchit1 (%rip)
 0x0f,0x18,0x35,0x00,0x00,0x00,0x00
+
+# Check that we correctly ignore a REX prefix that is not immediately before
+# the opcode.
+# CHECK: orw $25659, %ax
----------------
ondrasej wrote:

> No. `0x64` is the `FS` segment override prefix. It doesn't do anything here given we're not accessing memory. The opcode is actually `0x0d`.

Please add a link (resp. a reference) to the part of the Intel SDM where this is specified, in case this question is raised again.

https://github.com/llvm/llvm-project/pull/117299