Fixed some typos
ir: Use ARRAY_SIZE() instead of magic numbers.
doc: Correct typos in comment.
be: Factorise initialisation of backend info into the function be_info_init_irn().
be: Remove arch_set_irn_flags().

The only user should be an arch_add_irn_flags() instead.
It was harmless though, because no other flags were set.
be: Provide an inline implementation of arch_add_irn_flags().