This reverts commit 6c7c81e0.

Reason for revert: Dependent CL was reverted.

Original change's description:
> [turbofan] Properly zero-extend indices on 64-bit architectures.
> 
> This was an oversight from the previous CL. It doesn't really matter
> with the current code generation pattern, since the upper bits of the
> index will always be zero, but that might change in the future.
> 
> Bug: chromium:225811
> Change-Id: I568a0824cad9ce9b73a56decc15d146c7dc675a1
> Reviewed-on: https://chromium-review.googlesource.com/1174111
> Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
> Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#55104}

TBR=jarin@chromium.org,bmeurer@chromium.org

Change-Id: Ib344609b0c4734c6512e6be287a5b7f80bc3f603
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: chromium:225811
Reviewed-on: https://chromium-review.googlesource.com/1174232Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55109}