MIR Serialization: Serialize UsedPhysRegMask from the machine register info.

This commit serializes the UsedPhysRegMask register mask from the machine
register information class. The mask is serialized as an inverted
'calleeSavedRegisters' mask to keep the output minimal.

This commit also allows the MIR parser to infer this mask from the register
mask operands if the machine function doesn't specify it.

Reviewers: Duncan P. N. Exon Smith
llvm-svn: 244548