[dev.typeparams] cmd/compile: make types2 report better error for invalid untyped operation

This ports the fix in CL 328050 for typecheck to types2.

The fix is not identical, due to code structure differences between
typecheck and types2, but the idea is the same. We only do the untyped
conversion when both operands can be mixed.

Updates #46749

Change-Id: Ib2c63ba0d5dd8bf02318b1bfdfe51dcaeeeb7f82
Reviewed-on: https://go-review.googlesource.com/c/go/+/328053


Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>