[DebugInfo] Fix pr41180 : Loop Vectorization Debugify Failure

Bug: https://bugs.llvm.org/show_bug.cgi?id=41180

In the bug test case the debug location was missing for the cmp instruction in
the "middle block" BB. This patch fixes the bug by copying the debug location
from the cmp of the scalar loop's terminator branch, if it exists.

The patch also fixes the debug location on the subsequent branch instruction.
It was previously using the location of the of the original loop's pre-header
block terminator. Both of these instructions will now map to the source line of
the conditional branch in the original loop.

A regression test has been added that covers these issues.

Patch by Orlando Cazalet-Hyams!

Differential Revision: https://reviews.llvm.org/D59944

llvm-svn: 357499