[Attributor][FIX] Do not try to cast if a cast is not required

When we replace constant returns at the call site we did issue a cast in
the hopes it would be a no-op if the types are equal. Turns out that is
not the case and we have to check it ourselves first.

Reused an IPConstantProp test for coverage. No functional change to the
test wrt. IPConstantProp.