[CallSiteSplitting] Add debug location to created PHI nodes.

There are 2 cases when we create PHI nodes:
 * For the result of the call that was duplicated in the split blocks.
   Those PHI nodes should have the debug location of the call.

 * For values produced before the call. Those instructions need to be
   duplicated in the split blocks and the PHI nodes should have the
   debug locations of those instructions.

Fixes PR37962.

Reviewers: junbuml, gbedwell, vsk

Reviewed By: junbuml

Tags: #debug-info

Differential Revision: https://reviews.llvm.org/D51919

llvm-svn: 341970