AMDGPU/GlobalISel: Widen non-power-of-2 load results

Load extra bits if suitably aligned. This allows using widened
3-vector loads on SI, and fixes legalization for <9 x s32> (which LSV
apparently forms frequently on lowered kernel argument lists).

Fix incorrectly treating these as legal on SI. This should emit a
64-bit store and a 32-bit store.

I think all of the load and store rules are just about complete, but
due for a rewrite.