[SVE][CodeGen] Legalisation of unpredicated store instructions

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

Reviewers: sdesmalen, efriedma, david-arm

Reviewed By: david-arm

Subscribers: tschuett, hiraditya, psnobl, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D83041