[RISCV] Simplify some code in RISCVInstrInfo::verifyInstruction. NFCI

This code was written as if it lived in the MC layer instead of
the CodeGen layer. We get the MCInstrDesc directly from MachineInstr.
And we can use RISCVSubtarget::is64Bit instead of going to the
Triple.

Differential Revision: https://reviews.llvm.org/D133905