[GlobalISel] Add combine for PTR_ADD with regbanks

Combine two G_PTR_ADDs, but keep the register bank of the constant.
That way, the combine can be used in post-regbank-select combines.

Introduce two helper methods in CombinerHelper, getRegBank and
setRegBank that get and set an optional register bank to a register.
That way, they can be used before and after register bank selection.

Differential Revision: https://reviews.llvm.org/D103326