fold-const.c (fold <ABS_EXPR>): Re-fold the result of folding fabs(-x) into fabs(x).

fold-const.c (fold <ABS_EXPR>): Re-fold the result of folding fabs(-x) into fabs(x).

	* fold-const.c (fold <ABS_EXPR>):  Re-fold the result of folding
	fabs(-x) into fabs(x).  Use tree_expr_nonnegative_p to determine
	when the ABS_EXPR (fabs or abs) is not required.
  	(tree_expr_nonnegative_p): Move the logic that sqrt and exp are
	always nonnegative from fold to here.  Additionally, cabs and fabs
	are always non-negative, and pow and atan are non-negative if
	their first argument is non-negative.

	* builtins.c (fold_builtin_cabs): New function to fold cabs{,f,l}.
	Evaluate cabs of a constant at compile-time.  Convert cabs of a
	non-complex argument into fabs.  Convert cabs(z) into
	sqrt(z.r*z.r + z.i*z.i) at the tree-level with -ffast-math or
	-funsafe-math-optimizations or -ffast-math.
	(fold_builtin): Convert BUILT_IN_FABS{,F,L} into an ABS_EXPR.
	Fold BUILT_IN_CABS{,F,L} using fold_builtin_cabs.

	* gcc.dg/builtins-2.c: Add some more tests.
	* gcc.dg/builtins-18.c: New test case.
	* gcc.dg/builtins-19.c: New test case.

From-SVN: r67541