cmd/compile: absorb more Not ops into Neq* and Eq* ops

cmd/compile: absorb more Not ops into Neq* and Eq* ops

We absorbed Not into most integer comparisons but not into pointer
and floating point equality checks.

The new cases trigger more than 300 times during make.bash.

Change-Id: I77c6b31fcacde10da5470b73fc001a19521ce78d
Reviewed-on: https://go-review.googlesource.com/c/go/+/200618
Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>