R600/SI: Emit an error when attempting to spill VGPRs v4

I can't get VGPR spilling to work reliable, so for now just emit
an error when the register allocator tries to spill VGPRs.

v2:
  - Fix build
v3:
  - Added crash fix when spilling SPGRs
v4:
  - Use V_MOV_B32 as a dummy instruction instead of S_NOP

Patch by: Darren Powell

https://bugs.freedesktop.org/show_bug.cgi?id=75276

llvm-svn: 210588