prevent code bloat for for arrays

Author: tesuji

compiler/rustc_mir_transform/src/gvn.rs

@@ -1488,6 +1488,14 @@ impl<'tcx> VnState<'_, 'tcx> {
         if let Value::Constant { value, disambiguator: _ } = value
             && value.is_deterministic()
         {
+            // Prevent code bloat that makes
+            // `_2 = _1` now resolved to `_2 = <evaluated array>`.
+            if let Const::Val(_, ty) = value
+                && ty.is_array()
+                && self.rev_locals[index].len() > 1
+            {
+                return None;
+            }
             return Some(ConstOperand { span: DUMMY_SP, user_ty: None, const_: *value });
         }