[GlobalISel] Make G_PTR_ADD pattern matcher non-commutative.

G_PTR_ADD takes arguments of two different types, so it probably shouldn't be
considered commutative just on that basis. A recent G_PTR_ADD reassociation
optimization (https://reviews.llvm.org/D109528) can emit erroneous code if the
pattern matcher commutes the arguments; this can happen when the base pointer
was created by G_INTTOPTR of a G_CONSTANT and the offset register is variable.

This was discovered on the llvm-mos fork, but I added a failing test case that
should apply to AArch64 (and more generally).

Differential Revision: https://reviews.llvm.org/D114655