AMDGPU: Add DS append/consume intrinsics

Since these pass the pointer in m0 unlike other DS instructions, these
need to worry about whether the address is uniform or not. This
assumes the address is dynamically uniform, and just uses
readfirstlane to get a copy into an SGPR.

I don't know if these have the same 16-bit add for the addressing mode
offset problem on SI or not, but I've just assumed they do.

Also includes some misc. changes to avoid test differences between the
LDS and GDS versions.

llvm-svn: 352422