Recommit "[VPlan] Replace disjoint or with add instead of dropping disjoint. (#83821)"

Recommit with a fix for the use-after-free causing the revert.
This reverts the revert commit f872043e055f4163c3c4b1b86ca0354490174987.

Original commit message:

Dropping disjoint from an OR may yield incorrect results, as some
analysis may have converted it to an Add implicitly (e.g. SCEV used for
dependence analysis). Instead, replace it with an equivalent Add.

This is possible as all users of the disjoint OR only access lanes where
the operands are disjoint or poison otherwise.

Note that replacing all disjoint ORs with ADDs instead of dropping the
flags is not strictly necessary. It is only needed for disjoint ORs that
SCEV treated as ADDs, but those are not tracked.

There are other places that may drop poison-generating flags; those
likely need similar treatment.

Fixes https://github.com/llvm/llvm-project/issues/81872

PR: https://github.com/llvm/llvm-project/pull/83821

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