[coverage] do not invoke the normalizer when not necessary
[coverage] do not invoke the normalizer when not necessary

Summary:
Before, when calling `flow coverage` on non-covered files (e.g. non-@flow files)
we would still invoke the normalizer, even though we always returned `false` as
the result. With this change, we only invoke the normalizer when the file is covered.

Also optimizes puts calls to ALoc.to_loc inside the attendant folds.

Reviewed By: pakoito

Differential Revision: D13307520

fbshipit-source-id: 5706d2332aac953175f23b334c02a22212299f05

nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow