[AMDGPU] Introduce isBottomOfStack helper. NFC (#74288)

Introduce a helper to check if a function is at the bottom of the stack,
i.e. if it's an entry function or a chain function.
This was suggested in #71913.

Author: rovka

llvm/lib/Target/AMDGPU/AMDGPUMachineFunction.h

@@ -90,6 +90,11 @@ class AMDGPUMachineFunction : public MachineFunctionInfo {
 
   bool isChainFunction() const { return IsChainFunction; }
 
+  // The stack is empty upon entry to this function.
+  bool isBottomOfStack() const {
+    return isEntryFunction() || isChainFunction();
+  }
+
   bool hasNoSignedZerosFPMath() const {
     return NoSignedZerosFPMath;
   }

llvm/lib/Target/AMDGPU/SIMachineFunctionInfo.cpp

@@ -519,7 +519,7 @@ int SIMachineFunctionInfo::getScavengeFI(MachineFrameInfo &MFI,
                                          const SIRegisterInfo &TRI) {
   if (ScavengeFI)
     return *ScavengeFI;
-  if (isEntryFunction() || isChainFunction()) {
+  if (isBottomOfStack()) {
     ScavengeFI = MFI.CreateFixedObject(
         TRI.getSpillSize(AMDGPU::SGPR_32RegClass), 0, false);
   } else {

llvm/lib/Target/AMDGPU/SIRegisterInfo.cpp

@@ -503,7 +503,7 @@ Register SIRegisterInfo::getFrameRegister(const MachineFunction &MF) const {
   // functions, but never actually want to reference it when accessing our own
   // frame. If we need a frame pointer we use it, but otherwise we can just use
   // an immediate "0" which we represent by returning NoRegister.
-  if (FuncInfo->isEntryFunction() || FuncInfo->isChainFunction()) {
+  if (FuncInfo->isBottomOfStack()) {
     return TFI->hasFP(MF) ? FuncInfo->getFrameOffsetReg() : Register();
   }
   return TFI->hasFP(MF) ? FuncInfo->getFrameOffsetReg()
@@ -738,7 +738,7 @@ bool SIRegisterInfo::shouldRealignStack(const MachineFunction &MF) const {
 
   // FIXME: Should be able to specify the entry frame alignment per calling
   // convention instead.
-  if (Info->isEntryFunction() || Info->isChainFunction())
+  if (Info->isBottomOfStack())
     return false;
 
   return TargetRegisterInfo::shouldRealignStack(MF);
@@ -1649,7 +1649,7 @@ void SIRegisterInfo::buildSpillLoadStore(
         if (UseVGPROffset && ScratchOffsetReg) {
           MIB.addReg(ScratchOffsetReg);
         } else {
-          assert(FuncInfo->isEntryFunction() || FuncInfo->isChainFunction());
+          assert(FuncInfo->isBottomOfStack());
           MIB.addImm(0);
         }
       }
@@ -2424,7 +2424,7 @@ bool SIRegisterInfo::eliminateFrameIndex(MachineBasicBlock::iterator MI,
 
       bool IsMUBUF = TII->isMUBUF(*MI);
 
-      if (!IsMUBUF && !MFI->isEntryFunction() && !MFI->isChainFunction()) {
+      if (!IsMUBUF && !MFI->isBottomOfStack()) {
         // Convert to a swizzled stack address by scaling by the wave size.
         // In an entry function/kernel the offset is already swizzled.
         bool IsSALU = isSGPRClass(TII->getOpRegClass(*MI, FIOperandNum));