[LoopVectorize][X86] Clamp interleave factor if we have a known constant trip...

[LoopVectorize][X86] Clamp interleave factor if we have a known constant trip count that is less than VF*interleave

If we know the trip count, we should make sure the interleave factor won't cause the vectorized loop to exceed it.

Improves one of the cases from PR42674

Differential Revision: https://reviews.llvm.org/D65896

llvm-svn: 368215