Change all machine basic block modifier functions in MRegisterInfo to

return the number of instructions added to/removed from the basic block
passed as their first argument.

Note: This is only needed because we use a std::vector instead of an
ilist to keep MachineBasicBlock instructions. Inserting an instruction
to a MachineBasicBlock invalidates all iterators to the basic
block. The return value can be used to update an index to the machine
basic block instruction vector and circumvent the iterator elimination
problem but this is really not needed if we move to a better
representation.

llvm-svn: 9704