[AMDGPU] Merge the conditions used for deciding CS spills for amdgpu_cs_chain[_preserve] (llvm#109911)

Multiple conditions exist to decide whether callee save spills/restores
are required for amdgpu_cs_chain or amdgpu_cs_chain_preserve calling
conventions. This patch consolidates them all and moves to a single
place.

Change-Id: Iceb4609d0b9505ea3ec09023887d4e9261d8d033

Author: cdevadas

llvm/lib/Target/AMDGPU/SIFrameLowering.cpp

@@ -1601,16 +1601,10 @@ void SIFrameLowering::processFunctionBeforeFrameFinalized(
   SIMachineFunctionInfo *FuncInfo = MF.getInfo<SIMachineFunctionInfo>();
 
   // Allocate spill slots for WWM reserved VGPRs.
-  // For chain functions, we only need to do this if we have calls to
-  // llvm.amdgcn.cs.chain.
-  bool IsChainWithoutCalls =
-      FuncInfo->isChainFunction() && !MF.getFrameInfo().hasTailCall();
-  if (!FuncInfo->isEntryFunction() && !IsChainWithoutCalls) {
-    for (Register Reg : FuncInfo->getWWMReservedRegs()) {
-      const TargetRegisterClass *RC = TRI->getPhysRegBaseClass(Reg);
-      FuncInfo->allocateWWMSpill(MF, Reg, TRI->getSpillSize(*RC),
-                                 TRI->getSpillAlign(*RC));
-    }
+  for (Register Reg : FuncInfo->getWWMReservedRegs()) {
+    const TargetRegisterClass *RC = TRI->getPhysRegBaseClass(Reg);
+    FuncInfo->allocateWWMSpill(MF, Reg, TRI->getSpillSize(*RC),
+                               TRI->getSpillAlign(*RC));
   }
 
   const bool SpillVGPRToAGPR = ST.hasMAIInsts() && FuncInfo->hasSpilledVGPRs()

llvm/lib/Target/AMDGPU/SIMachineFunctionInfo.cpp

@@ -280,8 +280,10 @@ void SIMachineFunctionInfo::allocateWWMSpill(MachineFunction &MF, Register VGPR,
   // amdgpu_cs_chain_preserve calling convention and this is a scratch register.
   // We never need to allocate a spill for these because we don't even need to
   // restore the inactive lanes for them (they're scratchier than the usual
-  // scratch registers).
-  if (isChainFunction() && SIRegisterInfo::isChainScratchRegister(VGPR))
+  // scratch registers). We only need to do this if we have calls to
+  // llvm.amdgcn.cs.chain.
+  if (isChainFunction() && (SIRegisterInfo::isChainScratchRegister(VGPR) ||
+                            !MF.getFrameInfo().hasTailCall()))
     return;
 
   WWMSpills.insert(std::make_pair(