Avoid TransitionElementsKind when storing to objects which only differ
in holeyness of their elements kind. Instead go for polymorphic
CheckMaps, which can often by optimized and avoid the mutation of the
array map.

This generalizes the approach https://codereview.chromium.org/2836943003
which covered only element loads.

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

Review-Url: https://codereview.chromium.org/2836913004
Cr-Commit-Position: refs/heads/master@{#44828}