This lets us type the last VARIABLE. PrepareValueForWriteToTypedArray
still returns Node* for the non-templated version since it can return
Word32T or Float64T or Float32T or BigInt.

Bug: v8:6949
Change-Id: I90dee90d2e7eff08b1f69a57af371dec399b94c9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2282595
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Reviewed-by: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68714}