cmd/compile: optimize s==s for strings

cmd/compile: optimize s==s for strings

s==s is always true for strings. This comes up in NaN testing in
generic code, where we want x==x to compile completely away except for
float types.

Fixes #60777

Change-Id: I3ce054b5121354de2f9751b010fb409f148cb637
Reviewed-on: https://go-review.googlesource.com/c/go/+/503795
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>