[DebugInfo][RemoveDIs] Handle DPValues in LCSSA (#72996)

LCSSA needs to manually update dbg.value intrinsic users of Values that
are having LCSSA PHIs inserted -- instrument it to do the same for
DPValues, the replacement for dbg.values.

This patch also contains an opportunistic fix replacing
instruction-insertion with iterator-insertion, necessary for
communicating debug-info in the future.