[ValueTracking] allow non-canonical shuffles when computing signbits

This possibility is noted in D53987 for a different case,
so we need to adjust the existing code.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@345988 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed