[llvm] [RISCV][GISEL][NFC] Make MRI a member in RISCVInstructionSelector (PR #110926)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm approved this pull request.

LGTM, but should also avoid creating a new MachineIRBuilder for every instruction (if not just avoid using it altogether) 

https://github.com/llvm/llvm-project/pull/110926