[Regex] Avoid NFA machinery for fixed prefix chars (NFC)

Similarly to what backref() does, add an "easy path" to slow()
that can handle some non-branching cases, in particular simple
character matches.

This has the dual effect of reducing the number of characters we
need to match, and the number of states in the NFA.

This reduces FileCheck runtime on vloxseg.c from 17s to 12s on
my machine.