[LV] Clamp VF hint when unsafe

In the following loop the dependence distance is 2 and can only be
vectorized if the vector length is no larger than this.

  void foo(int *a, int *b, int N) {
    #pragma clang loop vectorize(enable) vectorize_width(4)
    for (int i=0; i<N; ++i) {
      a[i + 2] = a[i] + b[i];
    }
  }

However, when specifying a VF of 4 via a loop hint this loop is
vectorized. According to [1][2], loop hints are ignored if the
optimization is not safe to apply.

This patch introduces a check to bail of vectorization if the user
specified VF is greater than the maximum feasible VF, unless explicitly
forced with '-force-vector-width=X'.

[1] https://llvm.org/docs/LangRef.html#llvm-loop-vectorize-and-llvm-loop-interleave
[2] https://clang.llvm.org/docs/LanguageExtensions.html#extensions-for-loop-hint-optimizations

Reviewed By: sdesmalen, fhahn, Meinersbur

Differential Revision: https://reviews.llvm.org/D90687