[dev.typeparams] import operand.go changes from dev.go2go
[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 <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
Trust: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
Trust: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>

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