Reorganise optimisation level settings, and make the insn shrink pass optional (#1044)

This patch:

* removes the "default" opt level, on the basis that it has no definition and
  is referred to nowhere in the compiler.

* renames the "fastest" level to "none".  The resulting set of transformations
  is unchanged.

* renames the "best" level to "speed_and_size".  The resulting set of
  transformations is unchanged.

* adds a new level, "speed".  This is the same as "speed_and_size" except that
  it omits transformations aimed only at reducing code size.  Currently it
  omits only the insn shrinking pass.

cranelift/codegen/meta/src/shared/settings.rs

@@ -8,11 +8,12 @@ pub fn define() -> SettingGroup {
         r#"
         Optimization level:
 
-        - default: Very profitable optimizations enabled, none slow.
-        - best: Enable all optimizations
-        - fastest: Optimize for compile time by disabling most optimizations.
+        - none: Minimise compile time by disabling most optimizations.
+        - speed: Generate the fastest possible code
+        - speed_and_size: like "speed", but also perform transformations
+          aimed at reducing code size.
         "#,
-        vec!["default", "best", "fastest"],
+        vec!["none", "speed", "speed_and_size"],
     );
 
     settings.add_bool(