Makes RoundIntPtrToFloat64 return TNode<Float64T> instead of Node*.

Bug: v8:10155
Change-Id: I1edd5456b2b86b264b66eeab5e46ceb2a1f0170f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2064978
Commit-Queue: Dan Elphick <delphick@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66421}