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

[Pengcheng Wang via llvm-commits]

wangpc-pp wrote:

> The logic here looks sound, and this matches up with the comments on `TargetRegisterInfo::getRegPressureSetLimit`.
> 
> I'm not familiar with the register pressure-related callbacks - are there others that need to adapt for reserved registers in this way as well?

No other hook I think.

But I have to say, the situation here is a little dizzy: 
* We have two `getRegPressureSetLimit`: `RegisterClassInfo::getRegPressureSetLimit` and `TargetRegisterInfo::getRegPressureSetLimit`. And `RegisterClassInfo::getRegPressureSetLimit` is a wrapper of `TargetRegisterInfo::getRegPressureSetLimit`, with some logics to adjust the limit by reserved set:
https://github.com/llvm/llvm-project/blob/b2df0074134add80ba3a483a479601b00a9f9fc7/llvm/lib/CodeGen/RegisterClassInfo.cpp#L199-L234
* `RegisterClassInfo::getRegPressureSetLimit` (/wo reserved) is used in `RegPressureTracker`, `MachineScheduler`, etc.
* `TargetRegisterInfo::getRegPressureSetLimit` (/w reserved) is used directly in `MachineLICM`, `MachineSink`, `MachinePipeliner`, etc.

If we remove reserved registers in `TargetRegisterInfo::getRegPressureSetLimit`, than the limit calculated by `RegisterClassInfo::getRegPressureSetLimit` will be smaller than expected; but if we don't remove reserved registers (current implementation), `TargetRegisterInfo::getRegPressureSetLimit` returns incorrect limit that is bigger than expected.

Maybe we should change these direct users of `TargetRegisterInfo::getRegPressureSetLimit` to use `RegisterClassInfo::getRegPressureSetLimit`.

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