[AMDGPU][AsmParser/Disassembler] Correct A16 and G16 handling

A16 support for image instructions assembly/disassembly (gfx10) was missing

Also refactor MIMG op addr size calcs to common function

We'd got 3 places where the same operation was being done.

One test is now marked XFAIL until a related codegen patch is in place

Differential Revision: https://reviews.llvm.org/D102231

Change-Id: I7e86e730ef8c71901457855cba570581f4f576bb