[RISCV][NFC] Add RISCVSubtarget field to RISCVExpandPseudo and RISCVPreRAExpandPseudo

To my eye, it's cleaner to just get hold of STI in runOnMachineFunction
(as we do already for InstrInfo) and then accessing the field as needed
rather than to have repeated lookup code in the member functions or
helpers that need it.

Differential Revision: https://reviews.llvm.org/D155840