ARM64 backend, part 1 / 11: misc changes to existing code.

- Add a `simple_legalize()` function that invokes a predetermined set of
  legalizations, without depending on the details of the current
  backend design. This will be used by the new backend pipeline.

- Separate out `has_side_effect()` from the DCE pass. This will be used
  by the new backends' lowering code.

- Add documentation for the `Arm64Call` relocation type.

cranelift/codegen/meta/src/isa/arm64/mod.rs

@@ -54,7 +54,9 @@ pub(crate) fn define(shared_defs: &mut SharedDefinitions) -> TargetIsa {
     let mut a64 = CpuMode::new("A64");
 
     // TODO refine these.
+    let expand_flags = shared_defs.transform_groups.by_name("expand_flags");
     let narrow_flags = shared_defs.transform_groups.by_name("narrow_flags");
+    a64.legalize_monomorphic(expand_flags);
     a64.legalize_default(narrow_flags);
 
     let cpu_modes = vec![a64];