MIR Serialization: Serialize machine basic block operands.

This commit serializes machine basic block operands. The
machine basic block operands use the following syntax:

  %bb.<id>[.<name>]

This commit also modifies the YAML representation for the
machine basic blocks - a new, required field 'id' is added
to the MBB YAML mapping.

The id is used to resolve the MBB references to the
actual MBBs. And while the name of the MBB can be
included in a MBB reference, this name isn't used to
resolve MBB references - as it's possible that multiple
MBBs will reference the same BB and thus they will have the
same name. If the name is specified, the parser will verify
that it is equal to the name of the MBB with the specified id.

Reviewers: Duncan P. N. Exon Smith

Differential Revision: http://reviews.llvm.org/D10608

llvm-svn: 240792