Since we are converting Int64x2 to Int32x4, we should be truncating
the nodes, not sign-extending.

Bug: v8:10507
Change-Id: I09dabdcaaa378842ffb4da5505199c188b5101f1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2404751Reviewed-by: Bill Budge <bbudge@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69831}