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

gcc/
	* builtins.c (fold_builtin_powi): Delete.
	(fold_builtin_2): Handle constant powi arguments here.
	* match.pd: Add rules previously handled by fold_builtin_powi.

gcc/testsuite/
	* gcc.dg/builtins-52.c: Add -O to dg-options.

From-SVN: r229409