Only mark the last fallthrough control as deferred, otherwise the
splintering will ruin the code generation for the (maybe likely)
polymorphic cases.

Drive-by-fix: Reduce overall code duplication between JSLoadNamed
and JSStoreNamed specialization.

R=jarin@chromium.org
BUG=v8:4470
LOG=n

Review URL: https://codereview.chromium.org/1424733002

Cr-Commit-Position: refs/heads/master@{#31623}