[AArch64][GlobalISel] Add a new reassociation for G_PTR_ADDs.

G_PTR_ADD (G_PTR_ADD X, C), Y) -> (G_PTR_ADD (G_PTR_ADD(X, Y), C)

Improves CTMark -Os on AArch64:

Program            before after  diff
           sqlite3 286932 287024  0.0%
                kc 432512 432508 -0.0%
             SPASS 412788 412764 -0.0%
    pairlocalalign 249460 249416 -0.0%
            bullet 475740 475512 -0.0%
    7zip-benchmark 568864 568356 -0.1%
  consumer-typeset 419088 418648 -0.1%
        tramp3d-v4 367628 367224 -0.1%
          clamscan 383184 382732 -0.1%
            lencod 430028 429284 -0.2%
Geomean difference               -0.1%

Differential Revision: https://reviews.llvm.org/D109528