[GlobalISel] Propagate extends through G_PHIs into the incoming value blocks.

This combine tries to do inter-block hoisting of extends of G_PHIs, into the
originating blocks of the phi's incoming value. The idea is to expose further
optimization opportunities that are normally obscured by the PHI.

Some basic heuristics, and a target hook for AArch64 is added, to allow tuning.
E.g. if the extend is used by a G_PTR_ADD, it doesn't perform this combine
since it may be folded into the addressing mode during selection.

There are very minor code size improvements on AArch64 -Os, but the real benefit
is that it unlocks optimizations like AArch64 conditional compares on some
benchmarks.

Differential Revision: https://reviews.llvm.org/D95703