Converts NodeGenerator and SelectImpl to return TNode<T>.

Also removes redundant
PromiseBuiltinsAssembler::SetForwardingHandlerIfTrue overload.

Bug: v8:10021
Change-Id: I65e23fb208e4d9bf041e0f9c9b2ea113aeac58a5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1980576Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65594}