[RISCV] refactor VPatBinary (NFC)

Make it easier to reuse for intrinsic vrgatherei16
which needs to encode both LMUL & EMUL in the instruction name,
like PseudoVRGATHEREI16_VV_M1_M1 and PseudoVRGATHEREI16_VV_M1_M2.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D94951