[RISCV] Replace uses of RISCVOpcode<0b0010011> and RISCVOpcode<0b0011011> with...

[RISCV] Replace uses of RISCVOpcode<0b0010011> and RISCVOpcode<0b0011011> with existing named objects. NFC

These are already instantiated with names as OPC_OP_IMM and
OPC_OP_IMM_32.

Reviewed By: frasercrmck

Differential Revision: https://reviews.llvm.org/D115172