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

It was requested in
#110782 (comment)
that MRI be made a member of RISCVInstructionSelector.

RISCVInstructionSelector is created in the RISCVSubtarget, independent
of MachineFunction. So it cannot be passed by reference during
construction of RISCVInstructionSelector.

The MachineRegisterInfo object belongs to each MachineFunction, so
set it in setupMF.

Author: michaelmaitland