[JumpThreading][NFCI] Reuse existing DT instead of recomputation

This whole part with recomputation of BPI and BFI looks redundant,
and we tried to get rid of it in D124439. Unfortunately, it causes
some hard-to-reproduce failures due to invalid state of analysis.
Until this is investigated and fixed, let's try to reuse at least
part of available analyzes.

DT is available at this point, and there is no need to recompute it.

Please revert if you see it causing *any* behavior changes.