cmd/compile: add De Morgan's rewrite rule
Adds rules that rewrites statements such as ~P&~Q as ~(P|Q) and ~P|~Q as ~(P&Q), removing an extraneous instruction.
Change-Id: Icedb97df741680ddf9799df79df78657173aa500
GitHub-Last-Rev:
f22e2350c95e9052e990b2351c3c2b0af810e381
GitHub-Pull-Request: golang/go#60018
Reviewed-on: https://go-review.googlesource.com/c/go/+/493175
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Stefan M <st3f4nm4d4@gmail.com>
Reviewed-by: Keith Randall <khr@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>