Cranelift: remove non-egraphs optimization pipeline and use_egraphs option. (#6167)

* Cranelift: remove non-egraphs optimization pipeline and `use_egraphs` option.

This PR removes the LICM, GVN, and preopt passes, and associated support
pieces, from `cranelift-codegen`. Not to worry, we still have
optimizations: the egraph framework subsumes all of these, and has been
on by default since #5181.

A few decision points:

- Filetests for the legacy LICM, GVN and simple_preopt were removed too.
  As we built optimizations in the egraph framework we wrote new tests
  for the equivalent functionality, and many of the old tests were
  testing specific behaviors in the old implementations that may not be
  relevant anymore. However if folks prefer I could take a different
  approach here and try to port over all of the tests.

- The corresponding filetest modes (commands) were deleted too. The
  `test alias_analysis` mode remains, but no longer invokes a separate
  GVN first (since there is no separate GVN that will not also do alias
  analysis) so the tests were tweaked slightly to work with that. The
  egrpah testsuite also covers alias analysis.

- The `divconst_magic_numbers` module is removed since it's unused
  without `simple_preopt`, though this is the one remaining optimization
  we still need to build in the egraphs framework, pending #5908. The
  magic numbers will live forever in git history so removing this in the
  meantime is not a major issue IMHO.

- The `use_egraphs` setting itself was removed at both the Cranelift and
  Wasmtime levels. It has been marked deprecated for a few releases now
  (Wasmtime 6.0, 7.0, upcoming 8.0, and corresponding Cranelift
  versions) so I think this is probably OK. As an alternative if anyone
  feels strongly, we could leave the setting and make it a no-op.

* Update test outputs for remaining test differences.

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

@@ -53,19 +53,6 @@ pub(crate) fn define() -> SettingGroup {
         true,
     );
 
-    settings.add_bool(
-        "use_egraphs",
-        "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 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.
-        "#,
-        true,
-    );
-
     settings.add_bool(
         "enable_verifier",
         "Run the Cranelift IR verifier at strategic times during compilation.",