AArch64: don't assert inst within worst-case size when island emitted. (#4627)

We assert after emitting each instruction that its size was less than
the "worst-case size", which is used to determine when we need to
proactively emit an island so pending branch fixups don't go out of
bounds. However, the `EmitIsland` pseudo-inst itself can cause an
arbitrarily large island to be emitted; this should not have to fit
within the worst-case size (because island size is explicitly accounted
for by the threshold computation). This PR fixes the assert accordingly.

Fixes #4626.

cranelift/codegen/src/isa/aarch64/inst/emit.rs

@@ -3140,7 +3140,13 @@ impl MachInstEmit for Inst {
         }
 
         let end_off = sink.cur_offset();
-        debug_assert!((end_off - start_off) <= Inst::worst_case_size());
+        debug_assert!(
+            (end_off - start_off) <= Inst::worst_case_size()
+                || matches!(self, Inst::EmitIsland { .. }),
+            "Worst case size exceed for {:?}: {}",
+            self,
+            end_off - start_off
+        );
 
         state.clear_post_insn();
     }