fix issue #3440: when assigning the full slice of a list, evaluate the rhs

before clearing the list

--HG--
branch : release-pypy3.7-v7.x
2 jobs for release-pypy3.7-v7.3.5rc1 in 7 minutes and 8 seconds
latest
Status Name Job ID Coverage
  Test
passed check_rpython_annotations #192395

00:07:08

passed hpy_tests #192396

00:06:31