Remove debug location from common tail when tail-merging

The branch folding pass tail merges blocks into a common-tail.  However, the
tail retains the debug information from one of the original inputs to the
merge (chosen randomly).  This is a problem for sampled-based PGO, as hits
on the common-tail will be attributed to whichever block was chosen,
irrespective of which path was actually taken to the common-tail.

This patch fixes the issue by nulling the debug location for the common-tail.

Differential Revision: https://reviews.llvm.org/D25742

llvm-svn: 285093