MIR Serialization: Serialize simple MachineRegisterInfo attributes.

This commit serializes the 3 scalar boolean attributes from the
MachineRegisterInfo class: IsSSA, TracksRegLiveness, and
TracksSubRegLiveness. These attributes are serialized as part
of the machine function YAML mapping.

Reviewers: Duncan P. N. Exon Smith

Differential Revision: http://reviews.llvm.org/D10618

llvm-svn: 240579