[RFA] Fix Ada bootstrap (was: 16: EH redirection)

[Jan Hubicka]

> This is the patch I am testing (x86_64-linux) and will commit once
> passes.  The problem here is that removing dead TRY region imply need to
> update prev_try pointers in the subregions.  This is actually ages old
> bug, I am surprised it did not lead to any wrong code issues.
> 
> There is still one ICE on ada testsuite in my run.  This one is caused
> by dominance calcuation being called in forprop2 on CFG containing dead
> code.  Seems weird since preceeding CCP calls cfgcleanup.

Actually it is RTL fwprop2  and problem seems to be introduced by fact
that CSE calls cleanup_cfg only conditionally while it can produce dead
BBs.

Steven, since I think you are responsible for the conditional cfgcleanup
here, do you have any preferrences how to fix this?

Honza