Enable -Zlint-mir by default for mir-opt tests

Author: tmiasko

Diff for: src/tools/compiletest/src/runtest.rs

@@ -2445,6 +2445,7 @@ impl<'test> TestCx<'test> {
                     "-Copt-level=1",
                     &zdump_arg,
                     "-Zvalidate-mir",
+                    "-Zlint-mir",
                     "-Zdump-mir-exclude-pass-number",
                 ]);
                 if let Some(pass) = &self.props.mir_unit_test {

Diff for: tests/mir-opt/reference_prop.rs

@@ -1,3 +1,4 @@
+// compile-flags: -Zlint-mir=no
 // unit-test: ReferencePropagation
 // needs-unwind