[ValueTracking] ICmp instructions propagate poison

This is a stripped down version of D19211, leaving out the questionable
"branching in poison is UB" bit.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@271150 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed