The monomorphic case already carefully ensures that we don't try to use
a regular elements load stub on string wrapper elements. The polymorphic
path must perform an equivalent check.
BUG=chromium:594955
LOG=n
R=verwaest@chromium.org
Review URL: https://codereview.chromium.org/1806543002
Cr-Commit-Position: refs/heads/master@{#34807}
