[AArch64][SVE] Remove LD1/ST1 dependency on llvm.masked.load/store

Summary:
The SVE masked load and store intrinsics introduced in D76688 rely on
common llvm.masked.load/store nodes. This patch creates new ISD nodes
for LD1(S) & ST1 to remove this dependency.

Additionally, this adds support for sign & zero extending
loads and truncating stores.

Reviewers: sdesmalen, efriedma, cameron.mcinally, c-rhodes, rengolin

Reviewed By: efriedma

Subscribers: tschuett, kristof.beyls, hiraditya, rkruppe, psnobl, danielkiss, andwar, cfe-commits, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78204