Allow merging FrameStates with different local types

Summary: We already union the local types in FrameState::merge(). So,
FrameState::compatible() should not need to check for equality
of local types.

Reviewed By: @ottoni

Differential Revision: D1399916
Expose single-block loops

Summary: Even with Eval.JitLoops=true, loops with a single region block weren't
being exposed.  This diff adds those backedges if appropriate.

Reviewed By: @alexmalyshev

Differential Revision: D1431761