[PATCH] D109214: [DAG] Fold setcc eq with ashr to compare to zero.

[Dave Green via Phabricator via llvm-commits]

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Pulled out of D109149 <https://reviews.llvm.org/D109149>, this folds `set_cc seteq (ashr X, BW-1), -1` -> `set_cc setlt X, 0` to prevent some regressions later on when folding `select_cc setgt X, -1, C, ~C` -> `xor (ashr X, BW-1), C`


https://reviews.llvm.org/D109214

Files:
  llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
  llvm/test/CodeGen/AArch64/select-constant-xor.ll
  llvm/test/CodeGen/AMDGPU/select-constant-xor.ll
  llvm/test/CodeGen/ARM/select-constant-xor.ll
  llvm/test/CodeGen/PowerPC/select-constant-xor.ll
  llvm/test/CodeGen/RISCV/select-constant-xor.ll
  llvm/test/CodeGen/X86/select-constant-xor.ll

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