[DebugInfo][RemoveDIs] "Final" cleanup for non-instr debug-info (#79121)

Here's a raft of minor fixes for the RemoveDIs project that's replacing
dbg.value intrinsics with DPValue objects, all IMO trivial:
 * When inserting functions or blocks and calling setIsNewDbgInfoFormat,
   do that after setting the Parent pointer, just in case conversion from
   (or to) dbg.value mode is triggered.
 * When transferring DPValues from an empty range in a splice call, don't
   transfer if there are no DPValues attached to the source block at all.
 * stripNonLineTableDebugInfo should drop DPValues.
 * In insertBefore, don't try to transfer DPValues if there aren't any.