cmd/compile: move validation from unary/binaryOp to typecheck

CL 254400 makes typecheck set untyped type correctly. We now have enough
information to check valid operators for a type in typecheck.

Passes toolstash-check.

Change-Id: I01a7606ee6ce9964ec52430d53eaa886442bd17f
Reviewed-on: https://go-review.googlesource.com/c/go/+/255617


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