Remove instructions necessary for the old regalloc

cranelift/codegen/src/isa/aarch64/lower_inst.rs

@@ -2072,10 +2072,6 @@ pub(crate) fn lower_insn_to_regs<C: LowerCtx<I = Inst>>(
             });
         }
 
-        Opcode::Safepoint => {
-            panic!("safepoint instructions not used by new backend's safepoints!");
-        }
-
         Opcode::Trapz | Opcode::Trapnz | Opcode::ResumableTrapnz => {
             panic!("trapz / trapnz / resumable_trapnz should have been removed by legalization!");
         }