Target: Avoid getFirstTerminator() => pointer, NFC

Stop using an implicit conversion from the return of
MachineBasicBlock::getFirstTerminator to MachineInstr*.  In two cases,
directly dereference to a MachineInstr& since later code assumes it's
valid.  In a third case, change to an iterator since later code checks
against MachineBasicBlock::end.

Although the fix for the third case avoids undefined behaviour, I expect
this doesn't cause a functionality change in practice (since the basic
block already has a terminator).

llvm-svn: 274898