[RISCV] Detect more errors when parsing vsetvli in the assembler

-Reject an "mf1" lmul
-Make sure tail agnostic is exactly "tu" or "ta" not just that it starts with "tu" or "ta"
-Make sure mask agnostic is exactly "mu" or "ma" not just that it starts with "mu" or "ma"

Differential Revision: https://reviews.llvm.org/D92805