[SimplifyCFG] Merge debug locations when hoisting an instruction from a then/else branch. NFC.

Now that a new API to merge debug locations has been committed at r289661 (see
review D26256 for more details), we can use it to "improve" the code added by
revision r280995.

Instead of nulling the debugloc of a commoned instruction, we use the 'merged'
debug location. At the moment, this is just a no functional change since
function `DILocation::getMergedLocation()` is just a stub and would always
return a null location.

Differential Revision: https://reviews.llvm.org/D27804

llvm-svn: 289862