[AArch64][GlobalISel] Fold a chain of two G_PTR_ADDs of constant offsets.

E.g.
%addr1 = G_PTR_ADD %base, G_CONSTANT 20
%addr2 = G_PTR_ADD %addr1, G_CONSTANT 8
  -->
%addr2 = G_PTR_ADD %base, G_CONSTANT 28

Differential Revision: https://reviews.llvm.org/D72351