Make trailing operator handling consistent with L

Bug: 246391

Only ignore trailing BINARY operators when computing instant results.
We used to be much more aggressive.

Also ignore trailing binary operators when the user hits "=".

This makes us consistent with the L design and ensures that instant
results don't turn into errors when the user hits "=".

Change-Id: I260e95d152168b70774330ac95d5bc567cf79b3d