Fix class SCEVPredicate has virtual functions and accessible non-virtual destructor.

It is not enough to simply make the destructor virtual since there is a g++ 4.7
issue (see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53613) that throws the
error "looser throw specifier for ... overridding ~SCEVPredicate() noexcept".

llvm-svn: 254592