[LoopVectorizer][SVE] Vectorize a simple loop with with a scalable VF.

* Steps are scaled by `vscale`, a runtime value.
* Changes to circumvent the cost-model for now (temporary)
  so that the cost-model can be implemented separately.

This can vectorize the following loop [1]:

   void loop(int N, double *a, double *b) {
     #pragma clang loop vectorize_width(4, scalable)
     for (int i = 0; i < N; i++) {
       a[i] = b[i] + 1.0;
     }
   }

[1] This source-level example is based on the pragma proposed
separately in D89031. This patch only implements the LLVM part.

Reviewed By: dmgreen

Differential Revision: https://reviews.llvm.org/D91077

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