[llvm] [RISCV] Correct the limit of RegPresureSet `GPRAll` (PR #118473)

[Sam Elliott via llvm-commits]

================
@@ -934,3 +934,17 @@ bool RISCVRegisterInfo::getRegAllocationHints(
 
   return BaseImplRetVal;
 }
+
+unsigned RISCVRegisterInfo::getRegPressureSetLimit(const MachineFunction &MF,
+                                                   unsigned Idx) const {
+  if (Idx == RISCV::RegisterPressureSets::GPRAll) {
+    unsigned Reserved = 0;
+    BitVector ReservedRegs = getReservedRegs(MF);
+    for (MCPhysReg Reg = RISCV::X0_H; Reg <= RISCV::X31_H; Reg++)
+      if (ReservedRegs.test(Reg))
+        Reserved++;
+
+    return 32 - Reserved;
+  }
+  return RISCVGenRegisterInfo::getRegPressureSetLimit(MF, Idx);
----------------
lenary wrote:

The generated version in my build tree looks like the following:

```cpp
// Get the register unit pressure limit for this dimension.
// This limit must be adjusted dynamically for reserved registers.
unsigned RISCVGenRegisterInfo::
getRegPressureSetLimit(const MachineFunction &MF, unsigned Idx) const {
  static const uint8_t PressureLimitTable[] = {
    2,  	// 0: GPRC_and_SR07
    2,  	// 1: GPRX0
    2,  	// 2: SP
    2,  	// 3: GPRX7
    3,  	// 4: GPRX1
    8,  	// 5: FPR16C
    8,  	// 6: GPRF16C
    8,  	// 7: SR07
    8,  	// 8: VMV0
    14,  	// 9: GPRF16C_with_SR07
    16,  	// 10: GPRTC
    24,  	// 11: VRM8NoV0
    32,  	// 12: FPR16
    32,  	// 13: VM
    33,  	// 14: GPRAll
  };
  return PressureLimitTable[Idx];
}
```

So it is not correctly handling dynamically reserved registers.

https://github.com/llvm/llvm-project/pull/118473