[SVE][LoopVectorize] Add support for scalable vectorization of loops with vector reverse

This patch adds support for reverse loop vectorization.
It is possible to vectorize the following loop:
```
  for (int i = n-1; i >= 0; --i)
    a[i] = b[i] + 1.0;
```
with fixed or scalable vector.
The loop-vectorizer will use 'reverse' on the loads/stores to make
sure the lanes themselves are also handled in the right order.
This patch adds support for scalable vector on IRBuilder interface to
create a reverse vector. The IR function
CreateVectorReverse lowers to experimental.vector.reverse for scalable vector
and keedp the original behavior for fixed vector using shuffle reverse.

Differential Revision: https://reviews.llvm.org/D95363