[LV] Mark first-order recurrences as allowed exits

First-order recurrences require special treatment when they are live-out;
such treatment is provided by fixFirstOrderRecurrence(), so they should be
included in AllowedExit set.

(Should probably have been included originally in D16197.)

Fixes PR45526: AllowedExit set is used by prepareToFoldTailByMasking() to
check whether the treatment for live-outs also holds when folding the tail,
which is not (yet) the case for first-order recurrences.

Differential Revision: https://reviews.llvm.org/D78210