COND_EXPR lowering patch

[Zdenek Dvorak]

... is not quite ready yet, but I am getting closer.

The main showstopper seems to be the tree-ssa-dce; after this change
it cannot work in the current form (resp. it could, but since most of the
interesting control flow will then be expressed by explicit gotos, it
would be useless).

It was discussed before, but afair there was not any definitive
conclusion.  To me the following scheme seems to be the best:
The control structures removal part will only be activated with -O3
and will be done using regular control dependency analysis.

Rationale: this part of the optimization is rarely useful, and even if
it does something, ordinary dce together with jump threading does it
anyway (not completely, but for the most interesting cases it should
work).  It is also quite costly, which makes it a clear candidate for
-O3.  And this also seems to be the easiest way, since Jeff has the
requiered patch iirc.

Your opinions?

Zdenek