cmd/compile: clean up and optimize s390x multiplication rules

Some of the existing optimizations aren't triggered because they
are handled by the generic rules so this CL removes them. Also
some constraints were copied without much thought from the amd64
rules and they don't make sense on s390x, so we remove those
constraints.

Finally, add a 'multiply by the sum of two powers of two'
optimization. This makes sense on s390x as shifts are low latency
and can also sometimes be optimized further (especially if we add
support for RISBG instructions).

name                   old time/op  new time/op  delta
IntMulByConst/3-8      1.70ns ±11%  1.10ns ± 5%  -35.26%  (p=0.000 n=10+10)
IntMulByConst/5-8      1.64ns ± 7%  1.10ns ± 4%  -32.94%  (p=0.000 n=10+9)
IntMulByConst/12-8     1.65ns ± 6%  1.20ns ± 4%  -27.16%  (p=0.000 n=10+9)
IntMulByConst/120-8    1.66ns ± 4%  1.22ns ±13%  -26.43%  (p=0.000 n=10+10)
IntMulByConst/-120-8   1.65ns ± 7%  1.19ns ± 4%  -28.06%  (p=0.000 n=9+10)
IntMulByConst/65537-8  0.86ns ± 9%  1.12ns ±12%  +30.41%  (p=0.000 n=10+10)
IntMulByConst/65538-8  1.65ns ± 5%  1.23ns ± 5%  -25.11%  (p=0.000 n=10+10)

Change-Id: Ib196e6bff1e97febfd266134d0a2b2a62897989f
Reviewed-on: https://go-review.googlesource.com/c/go/+/248937


Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>