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