[AArch64][SVE] Implement structured store intrinsics

Summary:
This patch adds initial support for the following intrinsics:

    * llvm.aarch64.sve.st2
    * llvm.aarch64.sve.st3
    * llvm.aarch64.sve.st4

For storing two, three and four vectors worth of data. Basic codegen for
reg+immediate forms are implemented. Reg+reg addressing modes will be
addressed in a later patch.

These intrinsics are intended for use in the Arm C Language Extension
(ACLE).

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D75947