R600/SI: Fix a crash when spilling SGPRs

We need to make sure only one new instruction is added when spilling
otherwise the register allocator may crash.

This fixes a crash in the game Antichamber.

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