Use combined_fn in tree-vect-patterns.c

Use combined_fn in tree-vect-patterns.c

Another patch to extend uses of built_in_function to combined_fn,
this time in tree-vect-patterns.c.  The old code didn't handle the
long double pow variants, but I think that's because noone had a target
that would benefit rather than because the code would mishandle them.

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

gcc/
	* tree-vect-patterns.c: Include case-cfn-macros.h.
	(vect_recog_pow_pattern): Use combined_fn instead of built-in codes.

From-SVN: r230482