[DAG] combine assertsexts around a trunc

This was a suggested follow-up to:
D37017 / https://reviews.llvm.org/rL313577


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@315206 91177308-0d34-0410-b5e6-96231b3b80d8
8 files changed