[AArch64][SME] Fix chain for arm_locally_streaming functions.

The Chain wasn't set correctly in the DAG for functions marked
with aarch64_pstate_sm_body, which meant that SelectionDAG would
dead-code some of the CopyToReg's. This didn't show up in the
existing tests because all uses were in the same block, but when
adding some control-flow, suddenly things would break.

Reviewed By: kmclaughlin

Differential Revision: https://reviews.llvm.org/D136579