[CodeGen][SelectionDAG]Add new intrinsic experimental.vector.reverse

This patch adds  a new intrinsic experimental.vector.reduce that takes a single
vector and returns a vector of matching type but with the original lane order
 reversed. For example:

```
vector.reverse(<A,B,C,D>) ==> <D,C,B,A>
```

The new intrinsic supports fixed and scalable vectors types.
The fixed-width vector relies on shufflevector to maintain existing behaviour.
Scalable vector uses the new ISD node - VECTOR_REVERSE.

This new intrinsic is one of the named shufflevector intrinsics proposed on the
mailing-list in the RFC at [1].

Patch by Paul Walker (@paulwalker-arm).

[1] https://lists.llvm.org/pipermail/llvm-dev/2020-November/146864.html

Differential Revision: https://reviews.llvm.org/D94883