[ARM] Expand distributing increments to also handle existing pre/post inc instructions.

This extends the distributing postinc code in load/store optimizer to
also handle the case where there is an existing pre/post inc instruction,
where subsequent instructions can be modified to use the adjusted
offset from the increment. This can save us having to keep the old
register live past the increment instruction.

Differential Revision: https://reviews.llvm.org/D83377