[InstSimplify] Fold out-of-bounds shift to poison

[InstSimplify] Fold out-of-bounds shift to poison

Make InstSimplify return poison rather than undef for out-of-bounds
shifts, as specified by LandRef:

> If op2 is (statically or dynamically) equal to or larger than the
> number of bits in op1, this instruction returns a poison value.

Differential Revision: https://reviews.llvm.org/D93998