rtl: Enable the use of rtx values with int and mode attributes

The 'code' part of a 'define_code_attr' refers to the type of the key, in other
words, it uses a code_iterator to pick the 'value' from their (key "value") pair
list.

However, rtx_alloc_for_name requires a code_attribute to be used when the
'value' needs to be a type. In other words, no other type of attributes could be
used, before this patch, to produce a rtx typed 'value'.

This patch removes that restriction and allows the backend to use any kind of
attribute as long as that attribute always produces a valid code typed 'value'.

gcc/ChangeLog:

	* read-rtl.cc (rtx_reader::rtx_alloc_for_name): Allow all attribute
	types to produce code 'values'.
	(check_code_attribute): Rename ...
	(check_attribute_codes): ... to this.  And change comments to refer to
	* doc/md.texi: Add paragraph to document that you can use int and mode
	attributes to produce codes.