[InstCombine] fold lshr(trunc(lshr X, C1)) C2

Only the multi-use cases are changing here because there's
another fold that catches the simpler patterns.

But that other fold is the source of infinite loops when we
try to add D110170, so removing that is planned as a follow-up.

Attempt to show the general proof in Alive2:
https://alive2.llvm.org/ce/z/Ns1uS2

Note that the overshift fold-to-zero tests are not
currently handled by instsimplify. If they were, we
could assert that the shift amount sum is less than
the source bitwidth.