Based on D72931
This adds a new feature called A16 which is enabled for gfx10.
gfx9 keeps the R128A16 feature so it can share all the instruction encodings
with gfx7/8.
Differential Revision: https://reviews.llvm.org/D73956
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