Enable egraph-based optimization by default. (#5587)

This PR follows up on #5382 and #5391, which rebuilt the egraph-based optimization framework to be more performant, by enabling it by default.

Based on performance results in #5382 (my measurements on SpiderMonkey and bjorn3's independent confirmation with cg_clif), it seems that this is reasonable to enable. Now that we have been fuzzing compiler configurations with egraph opts (#5388) for 6 weeks, having fixed a few fuzzbugs that came up (#5409, #5420, #5438) and subsequently received no further reports from OSS-Fuzz, I believe it is stable enough to rely on.

This PR enables `use_egraphs`, and also normalizes its meaning: previously it forced optimization (it basically meant "turn on the egraph optimization machinery"), now it runs egraph opts if the opt level indicates (it means "use egraphs to optimize if we are going to optimize"). The conditionals in the top-level pass driver are a little subtle, but will get simpler once we can remove the non-egraph path (which we plan to do eventually!).

Fixes #5181.

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

@@ -58,10 +58,12 @@ pub(crate) fn define() -> SettingGroup {
         "Enable egraph-based optimization.",
         r#"
             This enables an optimization phase that converts CLIF to an egraph (equivalence graph)
-            representation, performs various rewrites, and then converts it back. This can result in
-            better optimization, but is currently considered experimental.
+            representation, performs various rewrites, and then converts it back. This should result in
+            better optimization, but the traditional optimization pass structure is also still
+            available by setting this to `false`. The `false` setting will eventually be
+            deprecated and removed.
         "#,
-        false,
+        true,
     );
 
     settings.add_bool(