The type of an intermediate multiply was wrong.  When that
intermediate multiply was spilled, the top 32 bits were lost.

Fixes #19153

Change-Id: Ib29350a4351efa405935b7f7ee3c112668e64108
Reviewed-on: https://go-review.googlesource.com/37212
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>