[Bug 75276] Implement VGPR Register Spilling

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

--- Comment #22 from farmboy0+freedesktop@googlemail.com --- Patch v4 fixes the crash in Antichamber: LLVM triggered Diagnostic Handler: SIInstrInfo::storeRegToStackSlot - Can't spill VGPR! LLVM triggered Diagnostic Handler: SIInstrInfo::loadRegToStackSlot - Can't retrieve spilled VGPR! radeon_llvm_compile: Processing Diag Flag LLVM failed to compile shader EE si_state.c:2133 si_shader_select - Failed to build shader variant (type=1) 1