[SCEV][LV] Invalidate LCSSA exit phis more thoroughly (#69909)

This an alternative to #69886. The basic problem is that SCEV can look
through trivial LCSSA phis. When the phi node later becomes non-trivial,
we do invalidate it, but this doesn't catch uses that are not covered by
the IR use-def walk, such as those in BECounts.

Fix this by adding a special invalidation method for LCSSA phis, which
will also invalidate all the SCEVUnknowns/SCEVAddRecExprs used by the
LCSSA phi node and defined in the loop.

We should probably also use this invalidation method in other places
that add predecessors to exit blocks, such as loop unrolling and loop
peeling.

Fixes #69097.
Fixes #66616.
Fixes #63970.

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