MIR Serialization: Serialize the register mask machine operands.

This commit implements serialization of the register mask machine
operands. This commit serializes only the call preserved register
masks that are defined by a target, it doesn't serialize arbitrary
register masks.

This commit also extends the TargetRegisterInfo class and TableGen so that
the users of TRI can get the list of all the call preserved register masks and
their names.

Reviewers: Duncan P. N. Exon Smith

Differential Revision: http://reviews.llvm.org/D10673

llvm-svn: 240966