DebugInfo: Teach Fast ISel to respect the debug location of comparisons in jumps

The use of the DbgLoc in FastISel is probably something we should fix.
It's prone to leaking the wrong location into instructions - we should
have a clear chain of custody from the debug location of an IR
Instruction to that of a MachineInstr to avoid such leakage.

llvm-svn: 227481