[InstCombine] matchRotate - add support for matching general funnel shifts...

[InstCombine] matchRotate - add support for matching general funnel shifts with constant shift amounts (PR46896)

First step towards extending the existing rotation support to full funnel shift handling now that the backend legalization support has improved.

This enables us to match the shift by constant cases, which are pretty trivial to expand again if necessary.

D88420 will add non-uniform support for funnel shifts as well once its been finalized.

Differential Revision: https://reviews.llvm.org/D88834