[DebugInfo][RemoveDIs] Allow speculative-DPMarker creation

There's good justification for having a function specifying "I need there to be
a marker here, so return the marker there or create a new one". This was going
to come later in the series, but it's starting to become necessary much eariler
alas.

Make use of it in spliceDebugInfo, where we can occasionally splice DPValues
onto the end() iterator of a block while it's been edited.