[RISCV] Add support for matching .vx and .vi forms of binary instructions for fixed vectors.

Unlike scalable vectors, I'm only using a ComplexPattern for
the immediate itself. The vmv_v_x is matched explicitly. We igore
the VL argument when matching a binary operator, but we do check
it when matching splat directly.

I left out tests for vXi64 as they fail on rv32 right now.

Reviewed By: frasercrmck

Differential Revision: https://reviews.llvm.org/D96365