Previously, without support for converting strings to numbers we'd
switch to megamorphic state and go to the runtime always to do the
conversion causing a performance cliff.

This patch improves the following js-perf-test scores:
Object-Lookup-String-Constant-BytecodeHandler: 4.25%
Object-Lookup-Index-String-BytecodeHandler: 5.41%

Bug: v8:9449
Change-Id: I63787fa84373fc946f1304b0141e48a52a1b4bcb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1690953Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Sathya Gunasekaran  <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63293}