Correctly ignore branch operators in the top-level parser when applicable.

An oversight in r320742 caused BREs to become sensitive to the branching operator prematurely, which caused
breakage in some limited situations -- namely, those that tried to use branching in a BRE. Most of these scenarios
had already been corrected beforehand to properly use gsed or grep for GNU extensions, so the damage is
slightly mitigated.

Reported by: antoine

Reported by: antoine
Approved by: emaste (mentor)
Differential Revision: https://reviews.freebsd.org/D11522