cmd/compile: remove memequal call from string compares in more cases

cmd/compile: remove memequal call from string compares in more cases

Add more rules to ensure that order doesn't matter.

Add memequal 0 rule.

Try to use a constant argument to memequal when one is available.

Fixes #59684

Change-Id: I36e85ffbd949396ed700ed6e8ec2bc3ae013f5d2
Reviewed-on: https://go-review.googlesource.com/c/go/+/485535
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>