[AMDGPU] Fix codegen of image intrinsics for g16 and a16

For gfx10 gradient (g16) and address (a16) can be independent. Previous
implementation assumed that a16 implied g16.

There are some other changes that fix the verification (as well as asm/disasm)
that are required for the included test to pass - the XFAIL will be removed in
those changes.

This also includes required fixes for GlobalISel

Differential Revision: https://reviews.llvm.org/D102066

Change-Id: I7d171cc90994de05f41669b66a6d0ffa2ed05d09