[AMDGPU] support image load/store a16

Our a16 support was only enabled for sample/gather and buffer
load/store, but not for image load/store operations (which take an i16
as the pixel index rather than a half).

Fix our isel lowering and add test cases to prove it out.

Differential Revision: https://reviews.llvm.org/D53750

llvm-svn: 345710