[PATCH][RFC] Add PAREN_EXPR, make flag_associative_math the default for Fortran

[Paolo Bonzini]

> so probably PAREN_EXPR needs to be ignored in some places, in this case
> it needs to be handled like a simple copy.

CCP (unlike VRP) is one of these.  Associativity does not change the 
"constant-ness" of a value.

Paolo