Let the SimplifiedOperatorReducer perform some strength reduction for
certain CheckTaggedSigned and CheckTaggedPointer inputs (reusing the
existing logic for ObjectIsSmi).

R=jarin@chromium.org
BUG=v8:5141

Review-Url: https://codereview.chromium.org/2080703006
Cr-Commit-Position: refs/heads/master@{#37167}