This removes the representation axis from the type of {Load/StoreField}
operators representing a property load/store. The representation would
be narrowed to {None} which causes problems for all places where we use
the type to reason about the value representation. Instead we should
fully switch to {MachineRepresentation}. This is just a stop-gap fix.

R=jarin@chromium.org
BUG=chromium:636716

Review-Url: https://codereview.chromium.org/2255533003
Cr-Commit-Position: refs/heads/master@{#38678}