[tcb] Merge PHI nodes -- 7.4% reduction of PHI nodes (Take 2)

[Diego Novillo]

Kazu,

I'm trying to understand what benefits do we get from this pass.  It
doesn't seem to me like increasing the in-degree of a basic block should
have any benefits.  Yes, we get to join more basic blocks, but at the
same time we increase the number of PHI arguments.

If we are missing optimization opportunities in the threader.  Then it
looks like we should be fixing that.  Adding more passes to cover for
shortcomings of existing passes is not usually a win.

And if the pass has negative compile time effects, then I'm definitely
not interested ;)


Diego.