cmd/compile: better error message for invalid untyped operation

For typed vs un-typed operation, the compiler do the conversion
un-conditionally, so if the operation is invalid, the error report is
pointed to the conversion, instead of the invalid operation itself.

To fix this, only do the conversion when the operations are valid
for both types.

Fixes #46749

Change-Id: Ib71c7bcd3ed5454e6df55b6a8db4e0f189259ba7
Reviewed-on: https://go-review.googlesource.com/c/go/+/328050


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>