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/src/ir/immediates.rs

@@ -57,6 +57,11 @@ impl Imm64 {
     pub fn wrapping_neg(self) -> Self {
         Self(self.0.wrapping_neg())
     }
+
+    /// Return bits of this immediate.
+    pub fn bits(&self) -> i64 {
+        self.0
+    }
 }
 
 impl Into<i64> for Imm64 {