R600/SI: add gather4 and getlod intrinsics (v3)

This contains all the previous patches + getlod support on top of it.
It doesn't use SDNodes anymore, so it's quite small.
It also adds v16i8 to SReg_128, which is used for the sampler descriptor.

Reviewed-by: Tom Stellard

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211228 91177308-0d34-0410-b5e6-96231b3b80d8