MIR Parser: make the machine instruction parsing interface more consistent. NFC.

MIR Parser: make the machine instruction parsing interface more consistent. NFC.

This commit refactors the interface for machine instruction parser. It adopts
the pattern of returning a bool and passing in the result in the first argument
that is used by the other parsing methods for the the method 'parse' and the
function 'parseMachineInstr'.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@241085 91177308-0d34-0410-b5e6-96231b3b80d8