[RemoveDIs] Use iterators for moving PHIs in loop-unroll-and-jam (#83003)

With no debug intrinsics, correctly identifying the start of a block
with iterators becomes important. We need to use the iterator-returning
methods here in loop-unroll-and-jam where we're shifting PHIs around.
Otherwise they can be inserted after debug-info records, leading to
debug-info attached to PHIs, which is ill formed.

Fixes #83000

Author: jmorse

llvm/lib/Transforms/Utils/LoopUnrollAndJam.cpp

@@ -473,9 +473,9 @@ llvm::UnrollAndJamLoop(Loop *L, unsigned Count, unsigned TripCount,
   };
   // Move all the phis from Src into Dest
   auto movePHIs = [](BasicBlock *Src, BasicBlock *Dest) {
-    Instruction *insertPoint = Dest->getFirstNonPHI();
+    BasicBlock::iterator insertPoint = Dest->getFirstNonPHIIt();
     while (PHINode *Phi = dyn_cast<PHINode>(Src->begin()))
-      Phi->moveBefore(insertPoint);
+      Phi->moveBefore(*Dest, insertPoint);
   };
 
   // Update the PHI values outside the loop to point to the last block