[ARM] Extend search for increment in load/store optimizer

Currently the findIncDecAfter will only look at the next instruction for
post-inc candidates in the load/store optimizer. This extends that to a
search through the current BB, until an instruction that modifies or
uses the increment reg is found. This allows more post-inc load/stores
and ldm/stm's to be created, especially in cases where a schedule might
move instructions further apart.

We make sure not to look any further for an SP, as that might invalidate
stack slots that are still in use.

Differential Revision: https://reviews.llvm.org/D95881