AMDGPU/SI: Fix encoding for FLAT_SCRATCH registers on VI

Summary:
These register has different encodings on CI and VI, so we add pseudo
FLAT_SCRACTH registers to be used before MC, and subtarget specific
registers to be used by the MC layer.

Reviewers: arsenm

Subscribers: arsenm, llvm-commits

Differential Revision: http://reviews.llvm.org/D15661

llvm-svn: 256178