[SimplifyCFG] Retain debug info when threading jumps with critical edges

Fixes bug 38023: https://bugs.llvm.org/show_bug.cgi?id=38023

The SimplifyCFG pass will perform jump threading in some cases where
doing so is trivial and would simplify the CFG. When folding a series
of blocks with redundant conditional branches into an unconditional "critical
edge" block, it does not keep the debug location associated with the previous
conditional branch.

This patch fixes the bug described by copying the debug info from the
old conditional branch to the new unconditional branch instruction, and
adds a regression test for the SimplifyCFG pass that covers this case.

Patch by Stephen Tozer!

Differential Revision: https://reviews.llvm.org/D59206

llvm-svn: 355833