R600/SI: Implement VGPR register spilling for compute at -O0 v3
VGPRs are spilled to LDS. This still needs more testing, but
we need to at least enable it at -O0, because the fast register
allocator spills all registers that are live at the end of blocks
and without this some future commits will break the
flat-address-space.ll test.
v2: Only calculate thread id once
v3: Move insertion of spill instructions to
SIRegisterInfo::eliminateFrameIndex()
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218348
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