[patch gimplifier]: Boolify more strict conditional expressions and transform simple form to binary

[Eric Botcazou]

> this patch converts TRUTH_AND_EXPR, TRUTH_OR_EXPR, and TRUTH_XOR_EXPR
> expressions on gimplification to their binary form.

What is it for?  This will redirect the compilation stream from proven paths to 
others so there must be a good reason to do it.  What's the effect on the code?

-- 
Eric Botcazou