[DebugInfo][RemoveDIs] Assert if we mix PHIs and debug-info (llvm#84054)

A potentially erroneous code construction with the work we've done to
remove debug intrinsics, is inserting PHIs into blocks when the position
hasn't been "sourced correctly". Specifically, if you have:

    %foo = PHI
    #dbg_value
    %bar = add i32...

And plan on inserting a new PHI, you have to use the iterator form of
`getFirstNonPHI` or getFirstInsertionPt (or begin()) to acquire an
iterator that tells the debug-info maintenance code "this is supposed to
be at the start of the block, put it in front of #dbg_value". We can
detect call-sites that aren't doing this at runtime, and should do with
this assertion. It might invalidate code that's doing something very
unexpected, like walking backwards to find a PHI, then going forwards,
then inserting: however that's just an inefficient way of calling
`getFirstNonPHI`.

llvm/lib/IR/Instruction.cpp

@@ -149,6 +149,18 @@ void Instruction::insertBefore(BasicBlock &BB,
   if (!InsertAtHead) {
     DPMarker *SrcMarker = BB.getMarker(InsertPos);
     if (SrcMarker && !SrcMarker->empty()) {
+      // If this assertion fires, the calling code is about to insert a PHI
+      // after debug-records, which would form a sequence like:
+      //     %0 = PHI
+      //     #dbg_value
+      //     %1 = PHI
+      // Which is de-normalised and undesired -- hence the assertion. To avoid
+      // this, you must insert at that position using an iterator, and it must
+      // be aquired by calling getFirstNonPHIIt / begin or similar methods on
+      // the block. This will signal to this behind-the-scenes debug-info
+      // maintenence code that you intend the PHI to be ahead of everything,
+      // including any debug-info.
+      assert(!isa<PHINode>(this) && "Inserting PHI after debug-records!");
       adoptDbgValues(&BB, InsertPos, false);
     }
   }