[InstCombine] dropRedundantMaskingOfLeftShiftInput(): propagate undef shift amounts
Summary:
When we do `ConstantExpr::getZExt()`, that "extends" `undef` to `0`,
which means that for patterns a/b we'd assume that we must not produce
any bits for that channel, while in reality we simply didn't care
about that channel - i.e. we don't need to mask it.
Reviewers: spatel
Reviewed By: spatel
Subscribers: hiraditya, llvm-commits
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D68239
llvm-svn: 373960