[PATCH] D142271: [ValueTracking] Add KnownBits patterns `xor(x, x - 1)` and `and(x, -x)` for knowing upper bits to be zero

[Noah Goldstein via Phabricator via llvm-commits]

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These two BMI pattern will clear the upper bits of result past the
first set bit. So if we know a single bit in `x` is set, we know that
`results[bitwidth - 1, log2(x) + 1] = 0`.

Alive2:
blsmsk: https://alive2.llvm.org/ce/z/a397BS
blsi: https://alive2.llvm.org/ce/z/tsbQhC


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D142271

Files:
  llvm/lib/Analysis/ValueTracking.cpp
  llvm/test/Analysis/ValueTracking/knownbits-bmi-pattern.ll
  llvm/test/Transforms/InstSimplify/ctpop-pow2.ll

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