[InstCombine] narrow abs with sign-extended input

In the motivating cases from https://llvm.org/PR48816 ,
we have a trailing trunc. But that is not required to
reduce the abs width:
https://alive2.llvm.org/ce/z/ECaz-p
...as long as we clear the int-min-is-poison bit (nsw).

We have some existing tests that are affected, and I'm
not sure what the overall implications are, but in general
we favor narrowing operations over preserving nsw/nuw.

If that causes problems, we could restrict this transform
based on type (shouldChangeType() and/or vector vs. scalar).

Differential Revision: https://reviews.llvm.org/D95235