[PATCH] D30311: [ValueTracking] Don't do an unchecked shift in ComputeNumSignBits

[Sanjoy Das via Phabricator via llvm-commits]

sanjoy created this revision.
Herald added a subscriber: mcrosier.

Previously we used to return a bogus result, 0, for IR like `ashr %val,
-1`.

I've also added an assert checking that `ComputeNumSignBits` at least
returns 1.  That assert found an already checked in test case where we
were returning a bad result for `ashr %val, -1`.

Fixes PR32045.


https://reviews.llvm.org/D30311

Files:
  lib/Analysis/ValueTracking.cpp
  test/Transforms/IndVarSimplify/pr32045.ll
  unittests/Analysis/ValueTrackingTest.cpp

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