[PATCH] D130956: [X86][MC] Always emit `rep` prefix for `bsf`

[Phoebe Wang via Phabricator via llvm-commits]

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`BMI` new instruction `tzcnt` has better performance than `bsf` on new
processors. Its encoding has a mandatory prefix '0xf3' compared to
`bsf`. If we force emit `rep` prefix for `bsf`, we will gain better
performance when the same code run on new processors.

GCC has already done this way: https://c.godbolt.org/z/6xere6fs1

Suggested by Linus in #56789


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D130956

Files:
  llvm/lib/Target/X86/X86InstrInfo.td
  llvm/test/MC/X86/I386-32.s
  llvm/test/MC/X86/I386-64.s
  llvm/test/MC/X86/x86-32-coverage.s

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