[GlobalISel] Add re-association combine for G_PTR_ADD to allow better addressing mode usage.

We're trying to match a few pointer computation patterns here for
re-association opportunities.
1) Isolating a constant operand to be on the RHS, e.g.:
   G_PTR_ADD(BASE, G_ADD(X, C)) -> G_PTR_ADD(G_PTR_ADD(BASE, X), C)

2) Folding two constants in each sub-tree as long as such folding
   doesn't break a legal addressing mode.
   G_PTR_ADD(G_PTR_ADD(BASE, C1), C2) -> G_PTR_ADD(BASE, C1+C2)

AArch64 code size improvements on CTMark with -Os:
Program              before  after   diff
 pairlocalalign      251048  251044 -0.0%
 consumer-typeset    421820  421812 -0.0%
 kc                  431348  431320 -0.0%
 SPASS               413404  413300 -0.0%
 clamscan            384396  384220 -0.0%
 tramp3d-v4          370640  370412 -0.1%
 lencod              432096  431772 -0.1%
 bullet              479400  478796 -0.1%
 sqlite3             288504  288072 -0.1%
 7zip-benchmark      573796  570768 -0.5%
 Geomean difference                 -0.1%

Differential Revision: https://reviews.llvm.org/D105069