[llvm] [MachinePipeliner] Use `RegisterClassInfo::getRegPressureSetLimit` (PR #119827)

[Pengcheng Wang via llvm-commits]

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@@ -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;
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wangpc-pp wrote:

> I think the heuristics will be less accurate with this change

I don't think so. These "fixed" register are also "reserved" registers, and "reserved" registers contain runtime fixed registers like those are reserved via `-ffixed-xx` options.

https://github.com/llvm/llvm-project/pull/119827