[LV] Make sure VF doesn't exceed compile time known TC

For the simple copy loop (see test case) vectorizer selects VF equal to 32 while the loop is known to have 17 iterations only. Such behavior makes no sense to me since such vector loop will never be executed. The only case we may want to select VF large than TC is masked vectoriztion. So I haven't touched that case.

Reviewed By: dmgreen

Differential Revision: https://reviews.llvm.org/D114528