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

[Phoebe Wang 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
----------------
phoebewang wrote:

I don't think ignore it is correct. The binutils doesn't recognize in this way either: https://godbolt.org/z/W3hjzanqT

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