Add size remarks to MachineFunctionPass

This adds per-function size remarks to codegen, similar to what we have in the
IR layer as of r341588. This only impacts MachineFunctionPasses.

This does the same thing, but for `MachineInstr`s instead of just
`Instructions`. After this, when a `MachineFunctionPass` modifies the number of
`MachineInstr`s in the function it ran on, you'll get a remark.

To enable this, use the size-info analysis remark as before.

llvm-svn: 341876