[dev.typeparams] import operand.go changes from dev.go2go
This involved some non-trivial changes from dev.go2go, due to the
refactoring of assignability in master.
Change-Id: I73d99053fc8b184ae79b7b8973bd15e69e50fe6b
Reviewed-on: https://go-review.googlesource.com/c/go/+/282119
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Robert Griesemer <gri@golang.org>
Trust: Robert Griesemer <gri@golang.org>
Trust: Robert Findley <rfindley@google.com>