[RemoveDIs] Replicate dbg intrinsic movement pattern in SelectOptimize (#81737)

Fix crash mentioned in comments on
d759618d.

The assertion being hit was complaining that we had dangling DPValues;
the DPValues attached to the terminator of StartBlock become dangling
after the terminator is erased, and they're never "flushed" back onto
the new terminator once it's added. Doing that makes the crash go away,
but doesn't replicate existing dbg.* behaviour. See the comment in the
patch.

This change both fixes the crash (because there are now no DPValues left
on the terminator to dangle) and replicates existing behaviour (moves
those DPValues down to the new block).