[SVE][CodeGen] Legalisation of unpredicated load instructions

Summary:
When splitting a load of a scalable type, the new address is
calculated in SplitVecRes_LOAD using a vscale and an add instruction.

This patch also adds a DAG combiner fold to visitADD for vscale:
 - Fold (add (vscale(C0)), (vscale(C1))) to (add (vscale(C0 + C1)))

Reviewers: sdesmalen, efriedma, david-arm

Reviewed By: david-arm

Subscribers: tschuett, hiraditya, rkruppe, psnobl, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D82792