only enable ConstProp on opt level 2

Author: Ze'ev Maor

Diff for: compiler/rustc_mir_transform/src/const_prop.rs

@@ -60,11 +60,8 @@ macro_rules! throw_machine_stop_str {
 pub struct ConstProp;
 
 impl<'tcx> MirPass<'tcx> for ConstProp {
-    fn is_enabled(&self, _sess: &rustc_session::Session) -> bool {
-        // FIXME(#70073): Unlike the other passes in "optimizations", this one emits errors, so it
-        // runs even when MIR optimizations are disabled. We should separate the lint out from the
-        // transform and move the lint as early in the pipeline as possible.
-        true
+    fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
+        sess.mir_opt_level() >= 2
     }
 
     #[instrument(skip(self, tcx), level = "debug")]
@@ -786,12 +783,6 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
 
     /// Returns `true` if and only if this `op` should be const-propagated into.
     fn should_const_prop(&mut self, op: &OpTy<'tcx>) -> bool {
-        let mir_opt_level = self.tcx.sess.mir_opt_level();
-
-        if mir_opt_level == 0 {
-            return false;
-        }
-
         if !self.tcx.consider_optimizing(|| format!("ConstantPropagation - OpTy: {:?}", op)) {
             return false;
         }