Fix bug in constant propagation.
Fix bug in constant propagation.

Functions with global effects were not considered const.
Intrinsics had a global side effect by default.

pombredanne/pythran,serge-sans-paille/pythran,hainm/pythran,hainm/pythran,artas360/pythran,pombredanne/pythran,pombredanne/pythran,artas360/pythran,artas360/pythran,serge-sans-paille/pythran,hainm/pythran,pbrunet/pythran,pbrunet/pythran,pbrunet/pythran