Follow-up cleanup for https://go-review.googlesource.com/17248:
Use properly typed local variable op now that that variable use
is not overloaded anymore.

Also: Remove unnecessary if stmt from common lexical path.

Change-Id: I984b0b346f3fdccd5aedc937330c0a5f99acf324
Reviewed-on: https://go-review.googlesource.com/17249Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>