[llvm][CodeGen] DAG Combiner folds for vscale.

Summary:
This patch simplifies the DAGs generated when using the intrinsic `@llvm.vscale.*` as follows:

* Fold (add (vscale * C0), (vscale * C1)) to (vscale * (C0 + C1)).
* Canonicalize (sub X, (vscale * C)) to (add X,  (vscale * -C)).
* Fold (mul (vscale * C0), C1) to (vscale * (C0 * C1)).
* Fold (shl (vscale * C0), C1) to (vscale * (C0 << C1)).

The test `sve-gep-ll` have been updated to reflect the folding introduced by this patch.

Reviewers: efriedma, sdesmalen, andwar, rengolin

Reviewed By: sdesmalen

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D74782