The pattern was generating zero-extended rather than sign-extended
CONST_INTs.

2019-09-23  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	PR target/91823
	* config/rs6000/altivec.md (altivec_copysign_v4sf3): Generate
	canonical CONST_INTs.  Use gen_rtvec.

From-SVN: r276055