[Reassociate] Use test cases that can actually be optimized to verify optional
flags are cleared.  The reassociation pass was just reordering the leaf nodes
in the previous test cases.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222250 91177308-0d34-0410-b5e6-96231b3b80d8