MachineInstr: Remove parameter from dump()

The primary use of the dump() functions in LLVM is for use in a
debugger. Unfortunately lldb does not seem to handle default arguments
so using `p SomeMI.dump()` fails and you have to type the longer `p
SomeMI.dump(nullptr)`. Remove the paramter to make the most common use
easy. (You can always construct something like `p
SomeMI.print(dbgs(),MyTII)` if you need more features).

Differential Revision: https://reviews.llvm.org/D29241

llvm-svn: 293440