[InstCombine] canonicalize funnel shift constant shift amount to be modulo bitwidth

The shift argument is defined to be modulo the bitwidth, so if that argument
is a constant, we can always reduce the constant to its minimal form to allow
better CSE and other follow-on transforms.

We need to be careful to ignore constant expressions here, or we will likely
infinite loop. I'm adding a general vector constant query for that case.

Differential Revision: https://reviews.llvm.org/D59374

llvm-svn: 356192