[MachinePipeliner] Skip reserved registers when computing register pressure (#120694)

We used to skip fixed registers, but fixed registers are not enough
because there are some runtime unusable registers like registers
reserved by `-ffixed-xxx` options.

Here we change to use reserved registers so that the estimated
pressure is more accurate.

Author: wangpc-pp

llvm/lib/CodeGen/MachinePipeliner.cpp

@@ -1283,9 +1283,9 @@ class HighRegisterPressureDetector {
     }
   }
 
-  // Return true if Reg is fixed one, for example, stack pointer
-  bool isFixedRegister(Register Reg) const {
-    return Reg.isPhysical() && TRI->isFixedRegister(MF, Reg.asMCReg());
+  // Return true if Reg is reserved one, for example, stack pointer
+  bool isReservedRegister(Register Reg) const {
+    return Reg.isPhysical() && MRI.isReserved(Reg.asMCReg());
   }
 
   bool isDefinedInThisLoop(Register Reg) const {
@@ -1311,7 +1311,7 @@ class HighRegisterPressureDetector {
         // because it's used only at the first iteration.
         if (MI.isPHI() && Reg != getLoopPhiReg(MI, OrigMBB))
           continue;
-        if (isFixedRegister(Reg))
+        if (isReservedRegister(Reg))
           continue;
         if (isDefinedInThisLoop(Reg))
           continue;
@@ -1423,7 +1423,7 @@ class HighRegisterPressureDetector {
 
     const auto InsertReg = [this, &CurSetPressure](RegSetTy &RegSet,
                                                    Register Reg) {
-      if (!Reg.isValid() || isFixedRegister(Reg))
+      if (!Reg.isValid() || isReservedRegister(Reg))
         return;
 
       bool Inserted = RegSet.insert(Reg).second;
@@ -1437,7 +1437,7 @@ class HighRegisterPressureDetector {
 
     const auto EraseReg = [this, &CurSetPressure](RegSetTy &RegSet,
                                                   Register Reg) {
-      if (!Reg.isValid() || isFixedRegister(Reg))
+      if (!Reg.isValid() || isReservedRegister(Reg))
         return;
 
       // live-in register