MIR Parser: adopt the 'maybeLex...' pattern. NFC.

This commit refactors the machine instruction lexer so that the lexing
functions use the 'maybeLex...' pattern, where they determine if they
can lex the current token by themselves.

Reviewers: Sean Silva

Differential Revision: http://reviews.llvm.org/D10817

llvm-svn: 241078