[patch, libfortran] Mark some codepaths as likely/unlikely

[Tobias Burnus]

Hi Thomas,

Thomas Koenig wrote:
> - A codepath that reaches generate_error() is assumed to be unlikely
>   

I think this is not needed. Functions which are marked as not returning
("__attribute__ ((noreturn))") are automatically regarded as unlikely
and I believe both in the library and in the front end, we properly tag
such functions.

By the way, also (pointer == NULL) is regarded as unlikely (which is not
necessarily true). I think there was something else, which I forgot,
however.

Tobias