fold-const.c (fold <MULT_EXPR>): Optimize both x*pow(x,c) and pow(x,c)*x as pow(x...

fold-const.c (fold <MULT_EXPR>): Optimize both x*pow(x,c) and pow(x,c)*x as pow(x,c+1) for constant values c.

	* fold-const.c (fold <MULT_EXPR>): Optimize both x*pow(x,c) and
	pow(x,c)*x as pow(x,c+1) for constant values c.  Optimize x*x
	as pow(x,2.0) when the latter will be expanded back into x*x.
	(fold <RDIV_EXPR>): Optimize pow(x,c)/x as pow(x,c-1).
	* builtins.c (expand_builtin_pow): Ignore flag_errno_math as
	pow can never set errno when used with an integer exponent.
	Always use expand_powi when exponent is -1, 0, 1 or 2.
	(fold_builtin): Don't rewrite pow(x,2.0) as x*x nor pow(x,-2.0)
	as 1.0/(x*x).  This avoids unbounded recursion as we now prefer
	the pow forms of these expressions.

	* gcc.dg/builtins-27.c: New test case.

From-SVN: r70030