[csa] Tnodify builtins-async-generator-gen.cc

Remove TaggedIsAsyncGenerator since all but one use is to generate
asserts that are handled automatically by TNodes. The remaining use is
then just inlined.

Also removes unused IsFastJSIterResult function.

Bug: v8:6949
Change-Id: Id5631586b7e4d4f43d352493a3e2638cf449665f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1849516
Commit-Queue: Dan Elphick <delphick@chromium.org>
Reviewed-by: Santiago Aboy Solanes <solanes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64186}