cmd/compile: distribute 8 and 16-bit multiplication

cmd/compile: distribute 8 and 16-bit multiplication

Expand the existing rule to cover 8 and 16 bit variants.

compilecmp linux/amd64:

time
time.parseStrictRFC3339.func1 80 -> 70  (-12.50%)
time.Time.appendStrictRFC3339.func1 80 -> 70  (-12.50%)
time.Time.appendStrictRFC3339 439 -> 428  (-2.51%)

time [cmd/compile]
time.parseStrictRFC3339.func1 80 -> 70  (-12.50%)
time.Time.appendStrictRFC3339.func1 80 -> 70  (-12.50%)
time.Time.appendStrictRFC3339 439 -> 428  (-2.51%)

linux/arm64:

time
time.parseStrictRFC3339.func1 changed
time.Time.appendStrictRFC3339.func1 changed
time.Time.appendStrictRFC3339 416 -> 400  (-3.85%)

time [cmd/compile]
time.Time.appendStrictRFC3339 416 -> 400  (-3.85%)
time.parseStrictRFC3339.func1 changed
time.Time.appendStrictRFC3339.func1 changed

Change-Id: I0ad3b2363a9fe8c322dd05fbc13bf151a146d8cb
Reviewed-on: https://go-review.googlesource.com/c/go/+/641756
Auto-Submit: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>