[InstCombine] matchFunnelShift - support non-uniform constant vector shift amounts (PR46895)

Complete basic PR46895 fixes by refactoring D87452/D88402 to allow us to match non-uniform constant values.

We still don't handle non-uniform vectors that contain undef elements, but that can wait until we have a decent generic mechanism for this.

Differential Revision: https://reviews.llvm.org/D88420