MIR Parser: wrap 'MBBSlots' from the MI parsing functions in a struct. NFC.

This commit modifies the interface for the machine instruction parsing
functions by wrapping the parameter 'MBBSlots' in a new structure called
'PerFunctionMIParsingState'. This change is useful as in the future I will be
able to pass new parameters to the machine instruction parser just by modifying
the 'PerFunctionMIParsingState' structure instead of adding a new parameter to
each function.

llvm-svn: 241607