We used to have a loop that it was used for JSReceivers. However, this
was not used as a proper loop since at most it would have two loop
iterations. Then, it could be changed to a Branch case.

Since I was refactoring the method, I also de-duplicated code by using
the common code from PlainPrimitiveNonNumberToNumber. In order to do so,
said method was renamed to TryPlain... and was reworked as well.

Bug: v8:6949, v8:10933
Change-Id: I860601a3b9e8bdeed052dcd237a767ac7ed80c92
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2435110
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70202}