C2: Don't create dummy Opaque1Node for outmost unswitched IfNode

In create_slow_version_of_loop(), C2 creates the outmost unswitched IfNode(i.e. **if(xx)**{ for{} }else{ for{} }) with a dummy opaque bool node as its condition input. After that, it sets the _prob of the outmost unswitched IfNode in do_unswitching().

I think we can merge these two steps into a single step, that is, create the outmost unswitched IfNode meanwhile setting its condition input, _prob and _fcnt w/ creating the dummy opaque bool node.