This fixes PR34681, where vectorization results in specializing the loop for the case where the unknown Stride N == 1 (where Stride happens to be equal to the loop iteration count N).
However, the vectorized loop will only be executed if the iteration count N>=VF, and since VF>1 in the vectorized loop, this implies that N > 1.
The two conditions cannot co-exist, so the vectorized loop body becomes dead code. (Eventually this dead code is identified and gets removed, but until then it can have some side-effects on the optimization passes on the way).
Instead, this patch avoids specialization of an unknown stride if we know that the "stride==1" predicate is going to contradict the loop-minimum-iteration-count guard. This gives the vectorizer a chance to try to vectorize the loop with the unknown stride, instead of falling directly to the scalar version.
While the motivation for this patch came from this vectorization scenario, it is relevant for any loop-optimization that wants to take advantage of the Stride==1 specialization when Stride is equal to the loop count, because optimizing a loop that executes at most one iteration probably doesn't make much sense, so we might as sell avoid this runtime guard.