[dev.regabi] cmd/compile: remove typ from AssignOpStmt
[dev.regabi] cmd/compile: remove typ from AssignOpStmt

Previous detached logic of typechecking AssignOpStmt from tcArith, the
typ field of it is not used anymore.

Pass toolstash -cmp.

Change-Id: I407507a1c4c4f2958fca4d6899875564e54bf1f5
Reviewed-on: https://go-review.googlesource.com/c/go/+/279443
Trust: Cuong Manh Le <54b4457f1dff6823379654eb5867596497ba782f@gmail.com>
Run-TryBot: Cuong Manh Le <54b4457f1dff6823379654eb5867596497ba782f@gmail.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Matthew Dempsky <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>

nawawi/go,nawawi/go,golang/go,nawawi/go,CAFxX/go,cloudflare/go,golang/go,CAFxX/go,CAFxX/go,CAFxX/go,cloudflare/go,cloudflare/go,cloudflare/go,CAFxX/go,cloudflare/go,CAFxX/go,nawawi/go,cloudflare/go,golang/go,cloudflare/go,CAFxX/go,nawawi/go,golang/go,CAFxX/go,golang/go,golang/go,nawawi/go,golang/go,nawawi/go,golang/go,cloudflare/go,nawawi/go