[AArch64][SVE] Asm: Support for structured LD2, LD3 and LD4 (scalar+scalar) load instructions.

Reviewers: rengolin, fhahn, samparker, SjoerdMeijer, javed.absar

Reviewed By: fhahn

Differential Revision: https://reviews.llvm.org/D46679

llvm-svn: 332442