* builtins.c (expand_builtin): Use expand_builtin_pow to expand
	calls for pow, powf, powl and their __builtin_ variants.
	(expand_builtin_pow): If the second argument is a constant
	integer and compiling with -ffast-math, use expand_powi to
	generate RTL if powi_cost is less than POWI_MAX_MULTS.
	(powi_cost): New function to return the number of multiplications
	necessary to evaluate an Nth power, for integer constant N.
	(expand_powi): New function to expand the RTL for evaluating
	the Nth power of a floating point value, for integer constant N.

	* doc/tm.texi (POWI_MAX_MULTS): Document new target macro.

	* gcc.dg/builtins-24.c: New test case.

From-SVN: r68401