The instruction selector assumed for Word32AtomicPairLoad node that if
there exists a Projection(1) user, then there also exists a
Projection(0) user. This, however, is not the case, because TurboFan
eliminates unreachable nodes. The missing projection node lead to a
failed DCHECK in the register allocator.

To fix the problem I use now the Word32AtomicPairLoad node directly to
allocate the register. On ia32 I stop additionally to allocate unneeded
temp registers.

R=gdeepti@chromium.org
CC=zhin@chromium.org

Bug: chromium:1042379
Change-Id: I79bd9f3f4672e147246a71c32b7c9b4dbd79b17f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2002547
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65912}