CodeGen: Use MachineInstr& in ScheduleDAGIntrs, NFC

Use MachineInstr& to avoid implicit conversions from
MachineBasicBlock::iterator to MachineInstr*.  In one case, this could
use a range-based for loop, but the other loops iterated in reverse
order.

One of the reverse-loops checked the MachineInstr* for nullptr, a
condition that is provably unreachable.  (And even if my proof has a
flaw, UBSan would catch the bug.)

llvm-svn: 274360