[RISCV] Refactor parseVTypeI and use ParseFail if we parsed more than one identifier.

Previously we lexed into a SmallVector and unlexed the tokens if
the parsing failed.

This patch gets rid of the SmallVector and the unlexing.

If the first token fails to parse, return MatchFail. This allows us
to fallback to parsing as an immediate. If we successfully parsed the
first token, use ParseFail if any later tokens fail to parse. This
avoids needing to UnLex the tokens.

I've used a state machine to keep track of what component we've
parsed so far.

Reviewed By: asb

Differential Revision: https://reviews.llvm.org/D150753