more precise type constraints in REDUCING

this better matches previous behavior when RESULT-TYPE is not supplied.
6 jobs for master in 13 minutes and 59 seconds (queued for 16 seconds)
Status Name Job ID Coverage
  Regression
passed ABCL regression tests #19137

00:08:22

passed CCL regression tests #19138

00:08:18

passed CMUCL regression tests #19140

00:02:51

passed Clasp regression tests #19139

00:10:40

passed ECL regression tests #19141

00:02:36

passed SBCL regression tests #19142

00:01:04