NVPTX: Avoid implicit iterator conversions, NFC

Avoid implicit conversions from MachineInstrBundleIterator to
MachineInstr* in the NVPTX backend, mainly by preferring MachineInstr&
over MachineInstr* when a pointer isn't nullable and using range-based
for loops.

There was one piece of questionable code in
NVPTXInstrInfo::AnalyzeBranch, where a condition checked a pointer
converted from an iterator for nullptr.  Since this case is impossible
(moreover, the code above guarantees that the iterator is valid), I
removed the check when I changed the pointer to a reference.

Despite that case, there should be no functionality change here.

llvm-svn: 274931