[AMDGPU] Assembler: Basic support for MIMG

Add parsing and printing of image operands. Matches legacy sp3 assembler.
Change image instruction order to have data/image/sampler operands in the beginning. This is needed because optional operands in MC are always last.
Update SITargetLowering for new order.
Add basic MC test.
Update CodeGen tests.

Review: http://reviews.llvm.org/D17574
llvm-svn: 261995