[AArch64] Add SVE2.1 target feature for Armv9-A 2022 Architecture Extension

[AArch64] Add SVE2.1 target feature for Armv9-A 2022 Architecture Extension

First patch in a series adding MC layer support for SVE2.1.

This patch adds the following feature:

sve2p1

Some of the existing SVE instructions added for SME are now
also available under the sve2p1 feature, which are now guarded
by the HasSVE2p1orSME predicate.

The reference can be found here:
https://developer.arm.com/documentation/ddi0602/2022-09

Differential Revision: https://reviews.llvm.org/D136352