[KnownBits] Return zero instead of unknown for always poison shifts

[KnownBits] Return zero instead of unknown for always poison shifts

For always poison shifts, any KnownBits return value is valid.
Currently we return unknown, but returning zero is generally more
profitable. We had some code in ValueTracking that tried to do this,
but was actually dead code.

Differential Revision: https://reviews.llvm.org/D150648