[PATCH] D120871: [X86] Emit REX prefix immediately before the opcode

[Amir Ayupov via Phabricator via llvm-commits]

Amir created this revision.
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Fix prefix emission order to emit REX immediately before the opcode
(SDM vol2, 2.1, Figure 2-1). 
According to SDM vol2 2.2.1, "Other placements are ignored".

This fix has a side effect of outputting segment override prefix in a different
order than previously (benigh effect).

Follow-up to https://reviews.llvm.org/D120592


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D120871

Files:
  llvm/lib/Target/X86/MCTargetDesc/X86MCCodeEmitter.cpp
  llvm/test/MC/Disassembler/X86/moffs.txt
  llvm/test/MC/X86/I186-32.s
  llvm/test/MC/X86/I186-64.s
  llvm/test/MC/X86/I386-32.s
  llvm/test/MC/X86/I386-64.s
  llvm/test/MC/X86/I86-32.s
  llvm/test/MC/X86/I86-64.s
  llvm/test/MC/X86/code16gcc.s
  llvm/test/MC/X86/index-operations.s
  llvm/test/MC/X86/x86-16.s

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