[dev.regabi] cmd/compile: improve ascompatee

order.go has already ordered function calls, so ascompatee only needs
to worry about expressions that might access a variable after it's
already been re-assigned. It already handles this, so the safeExpr
calls simply result in unnecessarily pessimistic code.

Does not pass toolstash -cmp, because it allows more efficient code
generation. E.g., cmd/go on linux/ppc64le is about 2kB smaller.

Change-Id: Idde0588eabe7850fa13c4e281fc46bbeffb4f68c
Reviewed-on: https://go-review.googlesource.com/c/go/+/281152


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>