In hexagon convertToHardwareLoop, don't deref end() iterator

In hexagon convertToHardwareLoop, don't deref end() iterator

In particular, check if MachineBasicBlock::iterator is end() before
using it to call getDebugLoc();

See also this thread on llvm-commits:
   http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20121112/155914.html

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@169634 91177308-0d34-0410-b5e6-96231b3b80d8