[InstCombine] fold cast of right-shift if high bits are not demanded (3rd try)

The first two tries at this were reverted because they caused an
infinite loop in instcombine.
That should be fixed after a series of patches that ended with
removing the faulty opposing transform:
3fabd98e

Original commit message:
(masked) trunc (lshr X, C) --> (masked) lshr (trunc X), C

Narrowing the shift should be better for analysis and can lead
to follow-on transforms as shown.

Attempt at a general proof in Alive2:
https://alive2.llvm.org/ce/z/tRnnSF

Here are a couple of the specific tests:
https://alive2.llvm.org/ce/z/bCnTp-
https://alive2.llvm.org/ce/z/TfaHnb

Differential Revision: https://reviews.llvm.org/D110170