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
Based on a discussion on D88783, if we're promoting a funnel shift to a width at least twice the size as the original type, then we can use the 'double shift' patterns (shifting the concatenated sources).
Differential Revision: https://reviews.llvm.org/D89139
Based on offline discussions regarding D89139 and D88783 - we want to make sure targets aren't doing anything particularly dumb
Tests copied from aarch64 which has a mixture of general, legalization and special case tests
Simon Pilgrim