Restrict the special handling of constant keys in ReduceKeyedAccess
to unique names. This eliminates two hurdles for concurrent optimization,
namely the need for ToName conversion and for string internalization.

Bug: v8:7790
Change-Id: Ifa2ff6ab1d5f3da1d27cca4dae3567733564801c
Reviewed-on: https://chromium-review.googlesource.com/c/1409168Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58779}