Changing rustc_codegen_llvm only will not rerun the tests

[nagisa]

Running the tests with

python x.py test --stage=1

changing the rustc_codegen_llvm and running the tests again with

python x.py test --stage=1

will ignore all the tests that have passed during the first run.

[cuviper]

It also seems like changes to rustc_codegen_llvm don't trigger the following stages of the compiler itself to rebuild.