[InstCombine] fold cast of right-shift if high bits are not demanded (2nd try)

[InstCombine] fold cast of right-shift if high bits are not demanded (2nd try)

The 1st try at this was reverted because it caused an infinite loop in instcombine.
That should be fixed after:
1cd6b44f267b

(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