When lowering DataField accesses, we should pay attention to the
MachineRepresentation reported by the AccessInfo. This doesn't
yet change the rest of the pipeline to take full advantage of
the representations.

Drive-by-fix: Make the code more robust, especially the part that
deals with mutable heap number access.

R=mvstanton@chromium.org
BUG=v8:5267,v8:5270

Review-Url: https://codereview.chromium.org/2334193002
Cr-Commit-Position: refs/heads/master@{#39381}