[PATCH] D152675: [DAGCombine] Use `IsKnownNeverZero` to see if we need zero-check in is_pow2 setcc patern

[Noah Goldstein via Phabricator via llvm-commits]

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`ctpop(X) eq/ne 1` is checking if X is a non-zero power of 2. Power of
2 check including zero is `(X & (X-1)) eq/ne 0` and unfortunately
there is no good pattern for checking a power of 2 while excluding
zero. So, when lowering `ctpop(X) eq/ne 1`, explicitly check
`IsKnownNeverZero(X)` to maybe be able to optimize out the extra zero
check.

We need this explicitly as DAGCombiner does not re-analyze provable
setcc nodes, and the middle-end never finds it beneficially to broaden
`ctpop(X) eq/ne 1` -> `ctpop(X) ule/ugt 1` (power of 2 including
zero).


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D152675

Files:
  llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
  llvm/test/CodeGen/X86/ispow2.ll

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