[AMDGPU] Rematerialize VGPR candidates when SGPR spills to VGPR over the VGPR limit

Before, when selecting candidates to rematerialize, we would only
consider SGPR candidates when there was an excess of SGPR registers.

Failing to eliminate the excess would result in spills to VGPRs.
This is normally not an issue, unless spilling to VGPRs results in
excess VGPRs.

This patch does 2 things:
* It relaxes the GCNRPTarget success criteria: now we accept regions
  where we spill SGPRs to VGPRs, as long as this does not end up in
  excess VGPRs.
* It changes isSaveBeneficial to consider the excess VGPRs (which
  includes the SGPRs that would be spilled to VGPR).

With these changes, the compiler rematerializes VGPRs when the excess
SGPRs would result in VGPR excess.

This has some unadressed flaws: we should attempt to rematerialize SGPRs
first in order to eliminate the SGPR excess that results in VGPR excess.

Solves SWDEV-549940

Author: jmmartinez

llvm/lib/Target/AMDGPU/GCNRegPressure.cpp

@@ -104,6 +104,7 @@ struct RegExcess {
   unsigned AGPR = 0;
 
   bool anyExcess() const { return SGPR || VGPR || ArchVGPR || AGPR; }
+  bool spillsToMemory() const { return VGPR || ArchVGPR || AGPR; }
 
   RegExcess(const MachineFunction &MF, const GCNRegPressure &RP,
             unsigned MaxSGPRs, unsigned MaxVGPRs) {
@@ -415,23 +416,20 @@ bool GCNRPTarget::isSaveBeneficial(Register Reg) const {
   const TargetRegisterInfo *TRI = MRI.getTargetRegisterInfo();
   const SIRegisterInfo *SRI = static_cast<const SIRegisterInfo *>(TRI);
 
+  RegExcess Excess(MF, RP, MaxSGPRs, MaxVGPRs);
+
   if (SRI->isSGPRClass(RC))
-    return RP.getSGPRNum() > MaxSGPRs;
-  unsigned NumVGPRs =
-      SRI->isAGPRClass(RC) ? RP.getAGPRNum() : RP.getArchVGPRNum();
-  // The addressable limit must always be respected.
-  if (NumVGPRs > MaxVGPRs)
-    return true;
-  // For unified RFs, combined VGPR usage limit must be respected as well.
-  return UnifiedRF && RP.getVGPRNum(true) > MaxUnifiedVGPRs;
+    return Excess.SGPR;
+
+  if (SRI->isAGPRClass(RC))
+    return Excess.AGPR;
+
+  return Excess.VGPR || Excess.ArchVGPR;
 }
 
 bool GCNRPTarget::satisfied() const {
-  if (RP.getSGPRNum() > MaxSGPRs || RP.getVGPRNum(false) > MaxVGPRs)
-    return false;
-  if (UnifiedRF && RP.getVGPRNum(true) > MaxUnifiedVGPRs)
-    return false;
-  return true;
+  RegExcess Excess(MF, RP, MaxSGPRs, MaxVGPRs);
+  return !Excess.spillsToMemory();
 }
 
 ///////////////////////////////////////////////////////////////////////////////