[AMDGPU] Extend the SI Load/Store optimizer to combine more things.

[AMDGPU] Extend the SI Load/Store optimizer to combine more things.

I've extended the load/store optimizer to be able to produce dwordx3
loads and stores, This change allows many more load/stores to be combined,
and results in much more optimal code for our hardware.

Differential Revision: https://reviews.llvm.org/D54042

llvm-svn: 348937