Enable GVN by default.

compiler/rustc_mir_transform/src/gvn.rs

@@ -109,7 +109,7 @@ pub struct GVN;
 
 impl<'tcx> MirPass<'tcx> for GVN {
     fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
-        sess.mir_opt_level() >= 4
+        sess.mir_opt_level() >= 2
     }
 
     #[instrument(level = "trace", skip(self, tcx, body))]

compiler/rustc_mir_transform/src/lib.rs

@@ -588,9 +588,9 @@ fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
             // destroy the SSA property. It should still happen before const-propagation, so the
             // latter pass will leverage the created opportunities.
             &separate_const_switch::SeparateConstSwitch,
-            &const_prop::ConstProp,
             &gvn::GVN,
             &simplify::SimplifyLocals::AfterGVN,
+            &const_prop::ConstProp,
             &dataflow_const_prop::DataflowConstProp,
             &const_debuginfo::ConstDebugInfo,
             &o1(simplify_branches::SimplifyConstCondition::AfterConstProp),