[SveEmitter] Use llvm.aarch64.sve.ld1/st1 for contiguous load/store builtins

This patch changes the codegen of the builtins for contiguous loads
to map onto the SVE specific IR intrinsics llvm.aarch64.sve.ld1/st1.

Reviewers: SjoerdMeijer, efriedma, kmclaughlin, rengolin

Reviewed By: efriedma

Tags: #clang

Differential Revision: https://reviews.llvm.org/D78673