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

[Sam Elliott via llvm-commits]

lenary 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?

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