[AArch64][GlobalISel] Fold selects fed by G_PTR_ADD

Similar to the case for G_ADD.

There was a function in CTMark/pairlocalalign which was missing this case,
causing GlobalISel to emit a add + csel when a csinc is all that is necessary.

https://godbolt.org/z/ax69E9

Minor code size improvements on CTMark at -Os.

Differential Revision: https://reviews.llvm.org/D96390