[RISCV] Merge FMV_H_X_RV32/FMV_H_X_RV64 into a single opcode. Same with FMV_X_ANYEXTH_RV32/RV64

Rather than having a different opcode for RV32 and RV64. Let's just say the integer type is XLenVT and use a single opcode for both modes.

Differential Revision: https://reviews.llvm.org/D92538