[MachinePipeliner] Use RegisterClassInfo::getRegPressureSetLimit

`RegisterClassInfo::getRegPressureSetLimit` is a wrapper of
`TargetRegisterInfo::getRegPressureSetLimit` with some logics to
adjust the limit by removing reserved registers.

It seems that we shouldn't use `TargetRegisterInfo::getRegPressureSetLimit`
directly, just like the comment "This limit must be adjusted
dynamically for reserved registers" said.

Thus we should use `RegisterClassInfo::getRegPressureSetLimit` and
remove replicated code.

Separate from #118787

Author: wangpc-pp

llvm/lib/CodeGen/MachinePipeliner.cpp

@@ -1326,48 +1326,7 @@ class HighRegisterPressureDetector {
   // Calculate the upper limit of each pressure set
   void computePressureSetLimit(const RegisterClassInfo &RCI) {
     for (unsigned PSet = 0; PSet < PSetNum; PSet++)
-      PressureSetLimit[PSet] = TRI->getRegPressureSetLimit(MF, PSet);
-
-    // We assume fixed registers, such as stack pointer, are already in use.
-    // Therefore subtracting the weight of the fixed registers from the limit of
-    // each pressure set in advance.
-    SmallDenseSet<Register, 8> FixedRegs;
-    for (const TargetRegisterClass *TRC : TRI->regclasses()) {
-      for (const MCPhysReg Reg : *TRC)
-        if (isFixedRegister(Reg))
-          FixedRegs.insert(Reg);
-    }
-
-    LLVM_DEBUG({
-      for (auto Reg : FixedRegs) {
-        dbgs() << printReg(Reg, TRI, 0, &MRI) << ": [";
-        for (MCRegUnit Unit : TRI->regunits(Reg)) {
-          const int *Sets = TRI->getRegUnitPressureSets(Unit);
-          for (; *Sets != -1; Sets++) {
-            dbgs() << TRI->getRegPressureSetName(*Sets) << ", ";
-          }
-        }
-        dbgs() << "]\n";
-      }
-    });
-
-    for (auto Reg : FixedRegs) {
-      LLVM_DEBUG(dbgs() << "fixed register: " << printReg(Reg, TRI, 0, &MRI)
-                        << "\n");
-      for (MCRegUnit Unit : TRI->regunits(Reg)) {
-        auto PSetIter = MRI.getPressureSets(Unit);
-        unsigned Weight = PSetIter.getWeight();
-        for (; PSetIter.isValid(); ++PSetIter) {
-          unsigned &Limit = PressureSetLimit[*PSetIter];
-          assert(
-              Limit >= Weight &&
-              "register pressure limit must be greater than or equal weight");
-          Limit -= Weight;
-          LLVM_DEBUG(dbgs() << "PSet=" << *PSetIter << " Limit=" << Limit
-                            << " (decreased by " << Weight << ")\n");
-        }
-      }
-    }
+      PressureSetLimit[PSet] = RCI.getRegPressureSetLimit(PSet);
   }
 
   // There are two patterns of last-use.