cmd/compile: add missing condition in SBFX -> SBFIZ opt

Only mattered today in practice with -N,
because constant folding masked it.

Appears to have been overlooked in CL 602935.

Change-Id: I50131cde4add88d538717bdd151ced7d5018d46a
Reviewed-on: https://go-review.googlesource.com/c/go/+/785260
Reviewed-by: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Mark Freeman <markfreeman@google.com>
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>