The AtomicNarrow operations are currently used for wider 64-bit
operations, that only operate on 32-bits of data or less
(Ex:I64AtomicAdd8U). Removing these because this can be handled
in int64-lowering by zeroing the higher order node.
Explicitly zeroing these in code-gen is not
required because -

 - The spec requires only the data exchange to be atomic, for narrow
   ops this uses only the low word.
 - The return values are not in memory, so are not visible to other
   workers/threads

BUG:v8:6532

Change-Id: I90a795ab6c21c70cb096f59a137de653c9c6a178
Reviewed-on: https://chromium-review.googlesource.com/1194428Reviewed-by: Ben Titzer <titzer@chromium.org>
Reviewed-by: Ben Smith <binji@chromium.org>
Commit-Queue: Deepti Gandluri <gdeepti@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55499}