SI-7877 Only look for unapplies in term trees

Since Scala 2.10.2, the enclosed test case has crashed
in the backend. Before, we correctly rejected this pattern match.

My bisection landed at a merge commit f16f4ab157, although both
parents were good. So I don't quite trust that.

I do think the regression stems from the changes to allow:

    case rx"AB(.+)" =>

Examples of this are in run/t7715.scala.

This commit limits the search for extractors to cases where the
function within the Apply is a term tree.