For polymorphic loads that refer to the same underlying field, but
differ in either representation or field maps, try harder to merge
them into a simple field load by going to Tagged representation or
throwing away the field maps (this is essentially what Crankshaft
is doing).

R=jarin@chromium.org
BUG=v8:5267

Review-Url: https://codereview.chromium.org/2836893002
Cr-Commit-Position: refs/heads/master@{#44831}