Fixes opt/localopts2

Because of Phis blocks cannot be equivalent. The cfopt must do all
block merging.

For example, consider the case where block A has one predecessor
block B, which contains a Jmp. Naively, we might exchange(A,B).
However, A might contain a Phi with one operand, then the graph
is broken afterwards, since we move the Phi into B as well.