[RISCV] Teach needVSETVLIPHI to handle mask register instructions.

This handles the case where the mask register instruction input
comes from a Phi of vsetvlis. If the VLMAX is the same as the VLMAX
required by the mask register instruction, we can avoid a vsetvli.

Reviewed By: frasercrmck

Differential Revision: https://reviews.llvm.org/D113204