[RISCV] Print human-readable VTYPE/SEW/LMUL in MIR

This patch adds custom MIR operand comments to VTYPE immediate operands
in VSETVLI instructions and SEW/LMUL operands in vector codegen pseudo
instructions. The result is intended to be more human-readable and
hopefully maintainable when working with MIR, particularly when
writing or reading test cases.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D124187