[tree-ssa] Thread jumps to blocks with PHIs

[Jan Hubicka]

> 
> This patch allows the jump threader to target a block with PHI nodes. 
> 
> Much like threading through a block with PHI nodes, we have to mark
> variables appearing in those PHI nodes as needing to be rewritten out
> of SSA form.
> 
> The only other complication is that we record the edge that leads to
> the final target rather than the final block.  This makes the code
> handle cases where out-of-ssa has to split the edge leading to
> the target block.  Whee.  About a 7% increase in the number of threading
> opportunities we find in the dominator jump threader and a tiny decrease
> in the number of threading opportunities in the RTL threader (which is
> precisely what we want -- to catch this stuff at the tree level rather
> than at the RTL level).

Just out of curiosity, how compares the number of threading opurtunities
discovered by each of those two beasts?

Honza