Improve and enable folding of conditional branches with tail calls.
- Make it so that conditional tail calls can be emitted even when there are multiple predecessors.
- Don't guard the transformation behind -Os. The rationale for guarding it was static-prediction can be affected by whether the branch is forward of backward. This is no longer true for almost any X86 cpus (anything newer than SnB) so is no longer a meaningful concern.
I think this flags turns to meaningless now. Either we leave code as was to match with SOM or we totally ignore it.