Add a combined_fn enum

Add a combined_fn enum

I'm working on a patch series that needs to be able to treat built-in
functions and internal functions in a similar way.  This patch adds a
new enum, combined_fn, that combines the two together.  It also adds
utility functions for seeing which combined_fn (if any) is called by
a given CALL_EXPR or gcall.

Tested on x86_64-linux-gnu, aarch64-linux-gnu and arm-linux-gnueabi.

gcc/
	* tree-core.h (internal_fn): Move immediately after the definition
	of built_in_function.
	(combined_fn): New enum.
	* tree.h (as_combined_fn, builtin_fn_p, as_builtin_fn)
	(internal_fn_p, as_internal_fn): New functions.
	(get_call_combined_fn, combined_fn_name): Declare.
	* tree.c (get_call_combined_fn): New function.
	(combined_fn_name): Likewise.
	* gimple.h (gimple_call_combined_fn): Declare.
	* gimple.c (gimple_call_combined_fn): New function.

From-SVN: r230472