Hg changeset 550: go.tools/ssa: fix builder crash in select { case n, _ := <-c: ... }.

Cause: emitExtract requires a type for each component of the
receive tuple; blank supplies no such type.

Solution: remove type parameter for emitExtract as it is no
longer needed: since rev b75cc03b4a56 it is always identical
to the tuple.Type().At(index).