[DebugInfo][InstrRef] Avoid some quadratic behaviour in LiveDebugVariables

This is a performance patch -- LiveDebugVariables can behave quadratically
if a lot of debug instructions are inserted back into the same place, and
we have to repeatedly step-over hte ones we've already inserted.

To get around it, whenever we insert a debug instruction at a slot index,
check whether there are more debug instructions to insert at this point,
and insert them too. That avoids the repeated lookup and stepping through.
It relies on the container for unlinked debug instructions being recorded
in-order, which is how LiveDebugVariables currently does it.

Differential Revision: https://reviews.llvm.org/D114587