MIR Serialization: Initial serialization of the machine operand target flags.

This commit implements the initial serialization of the machine operand target
flags. It extends the 'TargetInstrInfo' class to add two new methods that help
to provide text based serialization for the target flags.

This commit can serialize only the X86 target flags, and the target flags for
the other targets will be serialized in the follow-up commits.

Reviewers: Duncan P. N. Exon Smith
llvm-svn: 244185