In the special case of KeyedLoadIC, where the key is always undefined,
LowerCheckEqualsInternalizedString fails and triggers deoptimization
loops. It's because {exp}, which is undefined_string, does not match
{val}, which is undefined_value.

This patch drops the convertion from undefined_value to undefined_string
for undefined key in KeyedLoadIC. It makes undefined key be able to pass
LowerCheckEqualsInternalizedString and avoids the deoptimization loops.

Bug: 
Change-Id: Ie15358e705490fb2e4907f4d37a3ef7921f74844
Reviewed-on: https://chromium-review.googlesource.com/789613
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49779}