InstCombine: Avoid introducing poison values when lowering llvm.amdgcn.[us]bfe

Summary:
When the 3rd argument to these intrinsics is zero, lowering them
to shift instructions produces poison values, since we end up with
shift amounts equal to the number of bits in the shifted value.  This
means we can only lower these intrinsics if we can prove that the
3rd argument is not zero.

Reviewers: arsenm

Reviewed By: arsenm

Subscribers: bnieuwenhuizen, jvesely, wdng, nhaehnle, llvm-commits

Differential Revision: https://reviews.llvm.org/D53739

llvm-svn: 346422