cmd/compile: remove sign extension before MULW on riscv64
These sign extensions aren't necessary.
Change-Id: Id68ea596a18b30949d4605b2885f02e49e42d8e1
Reviewed-on: https://go-review.googlesource.com/c/go/+/699595 Reviewed-by: Keith Randall <khr@golang.org>
Auto-Submit: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Keith Randall <khr@google.com> Reviewed-by: Cherry Mui <cherryyz@google.com>