[IndVars] Recompute flags if needed in widenIVUse of IV increment. (#82352)

widenIVUse may hoist a wide induction increment and introduce new uses,
but does not recompute the wrap flags. In some cases this can make the
new uses of the wide IV inc more poisonous.

Update the code to recompute flags if needed when hoisting an IV. If
both the narrow and wide IV increment's flags match and we can re-use
the flags from the increments, there's no need to recompute the flags,
as the replacement won't make the new uses of the wide IV's increment
more poisonous.

Note that this also updates a stale comment which claimed that the widen
increment is only used if it dominates the new use.

The helper should also be used to guard the code added in da437330,
which I am planning on doing separately once the helper lands.

Fixes https://github.com/llvm/llvm-project/issues/82243.