Mostly constant folding rules, analogous to AMD64 ones. Along with
some simplifications.

Updates #15365.

Change-Id: If83bc1188bb05acb982ef3a1c21704c187e3eb24
Reviewed-on: https://go-review.googlesource.com/24210
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>