[llvm] [CodeGen] Refactor `determineCalleeSaves`. (PR #166763)

[Matt Arsenault via llvm-commits]

================
@@ -135,15 +119,81 @@ void TargetFrameLowering::determineCalleeSaves(MachineFunction &MF,
         MF.getFunction().hasFnAttribute(Attribute::NoUnwind) &&
         !MF.getFunction().hasFnAttribute(Attribute::UWTable) &&
         enableCalleeSaveSkip(MF))
-    return;
+    return nullptr;
+
+  // When interprocedural register allocation is enabled, callee saved register
+  // list should be empty, since caller saved registers are preferred over
+  // callee saved registers. Unless it has some risked CSR to be optimized out.
+  if (MF.getTarget().Options.EnableIPRA &&
+      isSafeForNoCSROpt(MF.getFunction()) &&
+      isProfitableForNoCSROpt(MF.getFunction()))
+    return TRI.getIPRACSRegs(&MF);
+  return MF.getRegInfo().getCalleeSavedRegs();
+}
 
+void TargetFrameLowering::determineUncondPrologCalleeSaves(
+    MachineFunction &MF, const MCPhysReg *CSRegs,
+    BitVector &UncondPrologCSRs) const {
   // Functions which call __builtin_unwind_init get all their registers saved.
-  bool CallsUnwindInit = MF.callsUnwindInit();
+  if (MF.callsUnwindInit()) {
+    for (unsigned i = 0; CSRegs[i]; ++i) {
+      unsigned Reg = CSRegs[i];
----------------
arsenm wrote:

```suggestion
      MCRegister Reg = CSRegs[i];
```

https://github.com/llvm/llvm-project/pull/166763