[AArch64][SME] Add support for arm_locally_streaming functions.

[AArch64][SME] Add support for arm_locally_streaming functions.

Functions with `aarch64_sme_pstatesm_body` will emit a SMSTART at the start
of the function, and a SMSTOP at the end of the function, such that all
operations use the right value for vscale.

Because the placement of these nodes is critically important (i.e. no
vscale-dependent operations should be done before SMSTART has been issued),
we require glueing the CopyFromReg to the Entry node such that we can
insert the SMSTART as part of that glued chain.

More details about the SME attributes and design can be found
in D131562.

Reviewed By: aemerson

Differential Revision: https://reviews.llvm.org/D131582