compiletest: run mir-opt tests with -C opt-level=1

The storage markers are removed at -C opt-level=0 and as a result output
of mir opt tests vary based on used optimization level. Use opt-level=1
for mir-opt tests to avoid the issue.

src/tools/compiletest/src/runtest.rs

@@ -1957,6 +1957,7 @@ impl<'test> TestCx<'test> {
             }
             MirOpt => {
                 rustc.args(&[
+                    "-Copt-level=1",
                     "-Zdump-mir=all",
                     "-Zmir-opt-level=3",
                     "-Zvalidate-mir",