[LoopSimplify] Set proper debug location in loop backedge blocks.

Set debug location for terminator instruction in loop backedge block
(which is an unconditional jump to loop header). We can't copy debug
location from original backedges, as there can be several of them,
with different debug info locations. So, we follow the approach of
SplitBlockPredecessors, and copy the debug info from first non-PHI
instruction in the header (i.e. destination block).

This is yet another change for PR23837.

llvm-svn: 240999