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

[Aiden Grossman via llvm-commits]

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@@ -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
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boomanaiden154 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`.

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