Hexagon: Avoid implicit iterator conversions, NFC

Avoid implicit iterator conversions from MachineInstrBundleIterator to
MachineInstr* in the Hexagon backend, mostly by preferring MachineInstr&
over MachineInstr* and switching to range-based for loops.

There's a long tail of API cleanup here, but I'm planning to leave the
rest to the Hexagon maintainers.  HexagonInstrInfo defines many of its
own predicates, and most of them still take MachineInstr*.  Some of
those actually check for nullptr, so I didn't feel comfortable changing
them to MachineInstr& en masse.

llvm-svn: 275142