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()
R600/SI: Fix SIRegisterInfo::getPhysRegSubReg()

Correctly handle special registers: EXEC, EXEC_LO, EXEC_HI, VCC_LO,
VCC_HI, and M0.  The previous implementation would assertion fail
when passed these registers.
R600/SI: Mark EXEC_LO and EXEC_HI as reserved

These registers can be allocated and used like other 32-bit registers,
but it seems like a likely source for bugs.
R600/SI: Fix the FixSGPRLiveRanges pass

The previous implementation was extending the live range of SGPRs
by modifying the live intervals directly.  This was causing a lot
of machine verification errors when the machine scheduler was enabled.

The new implementation adds pseudo instructions with implicit uses to
extend the live ranges of SGPRs, which works much better.
R600/SI: Move PHIs that define SGPRs to the VALU in most cases

This fixes a bug that is uncovered by a future commit and will
be tested by the test/CodeGen/R600/sgpr-control-flow.ll test case.
R600/SI: Enable selecting SALU inside branches

We can do this now that the FixSGPRLiveRanges pass is working.