[PATCH] D135022: [GlobalISel] Implement undef combines for funnel shifts

[Jessica Paquette via Phabricator via llvm-commits]

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AFAICT the way SDAG does this is:

  fshl (x, y, undef) -> x for scalars
  fshr (x, y, undef) -> y for scalars
  fshl (x, y, undef) -> undef for vectors
  fshr (x, y, undef) -> undef for vectors

https://gcc.godbolt.org/z/xq9PYfhYh

This should fix https://github.com/llvm/llvm-project/issues/57256


https://reviews.llvm.org/D135022

Files:
  llvm/include/llvm/CodeGen/GlobalISel/CombinerHelper.h
  llvm/include/llvm/Target/GlobalISel/Combine.td
  llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
  llvm/test/CodeGen/AArch64/GlobalISel/combine-undef-funnel-shift-amt.mir

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