Move signbit folds to match.pd

Move signbit folds to match.pd

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

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

gcc/testsuite/
	PR tree-optimization/68097
	* gcc.dg/torture/builtin-nonneg-1.c: Skip at -O0.  Add
	--param max-ssa-name-query-depth=3 to dg-options.

From-SVN: r229423