IBM Z: Change vector copysign to use bitwise operations

IBM Z: Change vector copysign to use bitwise operations

The vector copysign pattern incorrectly assumes that vector
if_then_else operates on bits, not on elements.  This can theoretically
mislead the optimizers.  Fix by changing it to use bitwise operations,
like commit 2930bb321794 ("PR94613: Fix vec_sel builtin for IBM Z") did
for vec_sel builtin.

gcc/ChangeLog:

2020-10-07  Ilya Leoshkevich  <iii@linux.ibm.com>

	* config/s390/s390-protos.h (s390_build_signbit_mask): New
	function.
	* config/s390/s390.c (s390_contiguous_bitmask_vector_p):
	Bitcast the argument to an integral mode.
	(s390_expand_vec_init): Do not call
	s390_contiguous_bitmask_vector_p with a scalar argument.
	(s390_build_signbit_mask): New function.
	* config/s390/vector.md (copysign<mode>3): Use bitwise
	operations.