[MachineCombiner] Extend reassociation logic to handle inverse instructions

[MachineCombiner] Extend reassociation logic to handle inverse instructions

Machine combiner supports generic reassociation only of associative and
commutative instructions, for example (A + X) + Y => (X + Y) + A. However, we
can extend this generic support to handle patterns like
(X + A) - Y => (X - Y) + A), where `-` is the inverse of `+`.
This patch adds interface functions to process reassociation patterns of
associative/commutative instructions and their inverse variants with minimal
changes in backends.

Differential Revision: https://reviews.llvm.org/D136754