[InstCombine] Fold `(X >>? C1) << C2` patterns to shift+bitmask (PR37872)

This is essentially finalizes a revert of rL155136,
because nowadays the situation has improved, SCEV can model
all these patterns well, and we canonicalize rotate-like patterns
into a funnel shift intrinsics in InstCombine.
So this should not cause any pessimization.

I've verified the canonicalize-{a,l}shr-shl-to-masking.ll transforms
with alive, which confirms that we can freely preserve exact-ness,
and no-wrap flags.

Profs:
* base: https://rise4fun.com/Alive/gPQ
* exact-ness preservation: https://rise4fun.com/Alive/izi
* nuw preservation: https://rise4fun.com/Alive/DmD
* nsw preservation: https://rise4fun.com/Alive/SLN6N
* nuw nsw preservation: https://rise4fun.com/Alive/Qp7

Refs. https://reviews.llvm.org/D46760