This change introduces the necessary conversion operators to convert
from Word64 to other representations (Tagged, Word32, Float64, etc.),
and plugs in the Word64 representation for NumberAdd/NumberSubtract,
such that TurboFan will go to Int64Add/Sub on 64-bit architectures
when the inputs and the output of the operation is in safe integer
range. This includes the necessary changes to the Deoptimizer to be
able to rematerialize Int64 values as Smi/HeapNumber when going back
to Ignition later.

This change might affect performance, although measurements indicate
that there should be no noticable performance impact.

The goal is to have TurboFan support Word64 representation to a degree
that changing the TypedArray length to an uint64_t (for 64-bit archs)
becomes viable and doesn't have any negative performance implications.
Independent of that we might get performance improvements in other areas
such as for crypto code later.

Bug: v8:4153, v8:7881, v8:8171, v8:8178
Design-Document: bit.ly/turbofan-word64
Change-Id: I29d56e2a31c1bae61d04a89d29ea73f21fd49c59
Cq-Include-Trybots: luci.chromium.try:linux_chromium_headless_rel
Reviewed-on: https://chromium-review.googlesource.com/1225709
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55937}