[NFC][Debuginfo][RemoveDIs] Switch an insertion to use iterators

With the soon-to-land new-debug-info storage model, it's going to be
important to use iterators for instruction insertion rather than
instruction pointers. This (single line in instcombine) is the last place
that trips up our internal testing for debug-info, where we insert a PHI
and it should be using an iterator.

Author: jmorse

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

@@ -4522,7 +4522,7 @@ bool InstCombinerImpl::run() {
           if (isa<PHINode>(I)) // PHI -> Non-PHI
             InsertPos = InstParent->getFirstInsertionPt();
           else // Non-PHI -> PHI
-            InsertPos = InstParent->getFirstNonPHI()->getIterator();
+            InsertPos = InstParent->getFirstNonPHIIt();
         }
 
         Result->insertInto(InstParent, InsertPos);