regexp/syntax: fix factoring of common prefixes in alternations

regexp/syntax: fix factoring of common prefixes in alternations

In the past, `a.*?c|a.*?b` was factored to `a.*?[bc]`. Thus, given
"abc" as its input string, the automaton would consume "ab" and
then stop (when unanchored) whereas it should consume all of "abc"
as per leftmost semantics.

Fixes #13812.

Change-Id: I67ac0a353d7793b3d0c9c4aaf22d157621dfe784
Reviewed-on: https://go-review.googlesource.com/18357
Reviewed-by: Russ Cox <rsc@golang.org>