This testcase was breaking because is_same::operator() is only used in 
constexpr evaluation, so cgraph decides it isn't used and throws it 
away.  Then mangling tries to use it for constexpr evaluation while 
re-instantiating F<A> under get_mostly_instantiated_function_type, and 
sadness ensues.

It occurred to me that we really ought to be able to avoid re-doing this 
instantiation, since DECL_TI_TEMPLATE of a function is a partially 
instantiated template that already has the type we want.  And indeed 
this seems to work well, even fixing a bug in mangling.

Tested x86_64-pc-linux-gnu, applying to trunk.