[PowerPC] 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.

Separate from llvm#118787

Author: wangpc-pp

Diff for: llvm/lib/Target/PowerPC/PPCInstrInfo.cpp

@@ -643,8 +643,8 @@ bool PPCInstrInfo::shouldReduceRegisterPressure(
   };
 
   // For now we only care about float and double type fma.
-  unsigned VSSRCLimit = TRI->getRegPressureSetLimit(
-      *MBB->getParent(), PPC::RegisterPressureSets::VSSRC);
+  unsigned VSSRCLimit =
+      RegClassInfo->getRegPressureSetLimit(PPC::RegisterPressureSets::VSSRC);
 
   // Only reduce register pressure when pressure is high.
   return GetMBBPressure(MBB)[PPC::RegisterPressureSets::VSSRC] >