[LV] Set SCEVCheckCond to nullptr whenever it was used.

Under some circumstances, SCEVExpander will insert new instructions when
expanding a predicate, but the final result of the expansion can be a
false constant.

In those cases, the expanded instructions may later be used by other
expansions, e.g. the trip count. This may trigger an assertion during
SCEVExpander cleanup. To avoid this, always mark the result as used.

Fixes #55100.