Replace addu with a sll instruction with a definite behavior (sll will discard the upper 32 bits of the 64 bits, then do sign extensions, with certain behavior). It won't have any UNPREDICTABLE expectation.

Fixes #30459

Change-Id: Id79085c28c5cc4f86939b4ef08ef4bff46077c45
GitHub-Last-Rev: 03569796a9a64ed6c7d56a5bca966fc62c89b4ff
GitHub-Pull-Request: golang/go#30461
Reviewed-on: https://go-review.googlesource.com/c/go/+/164758Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>