Use combined_fn in tree-ssa-reassoc.c

Use combined_fn in tree-ssa-reassoc.c

Another patch to extend uses of built_in_function to combined_fn, this time
in tree-ssa-reassoc.c.

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

gcc/
	* tree-ssa-reassoc.c: Include case-cfn-macros.h.
	(stmt_is_power_of_op): Use combined_fn instead of built-in codes.
	(decrement_power, acceptable_pow_call): Likewise.
	(attempt_builtin_copysign): Likewise.

From-SVN: r230480