[SelectionDAG] Optimize expansion for rotates/funnel shifts

If the type of a funnel shift needs to be expanded, expand it to two funnel shifts instead of regular shifts. For constant shifts, this doesn't make much difference, but for variable shifts it allows a more optimal lowering.

Also use the optimized funnel shift lowering for rotates.

Alive2: https://alive2.llvm.org/ce/z/TvHDB- / https://alive2.llvm.org/ce/z/yzPept

(Branched from D108058 as getting this completed should help unlock some other WIP patches).

Original Patch: @efriedma (Eli Friedman)

Differential Revision: https://reviews.llvm.org/D112443