[MachineOutliner] Refactor iterating over Candidate's instructions (#78972)

Make Candidate's front() and back() functions return references to
MachineInstr and introduce begin() and end() returning iterators, the
same way it is usually done in other container-like classes.

This makes possible to iterate over the instructions contained in
Candidate the same way one can iterate over MachineBasicBlock (note that
begin() and end() return bundled iterators, just like MachineBasicBlock
does, but no instr_begin() and instr_end() are defined yet).