[dev.typeparams] cmd/compile/internal/types2: review of predicates.go

Make predicates.go match the corresponding and reviewed go/types version.

The remaining diffs are due to the difference in the implementations
of the type conversion methods/functions:

$ diff $GOROOT/src/cmd/compile/internal/types2/predicates.go $GOROOT/src/go/types/predicates.go
7c7
< package types2
---
> package types
9a10
> 	"go/token"
32c33
< 	switch t := optype(typ.Under()).(type) {
---
> 	switch t := optype(typ).(type) {
63c64
< 	// set up. Must not call Basic()!
---
> 	// set up. Must not call asBasic()!
79c80
< 	t := typ.Basic()
---
> 	t := asBasic(typ)
85c86
< 	return typ.Interface() != nil
---
> 	return asInterface(typ) != nil
110c111
< 	if t := T.TypeParam(); t != nil && optype(t) == theTop {
---
> 	if t := asTypeParam(T); t != nil && optype(t) == theTop {
114c115
< 	switch t := optype(T.Under()).(type) {
---
> 	switch t := optype(T).(type) {
143c144
< 	switch t := optype(typ.Under()).(type) {
---
> 	switch t := optype(typ).(type) {
300,301c301,302
< 				check.completeInterface(nopos, x)
< 				check.completeInterface(nopos, y)
---
> 				check.completeInterface(token.NoPos, x)
> 				check.completeInterface(token.NoPos, y)

Change-Id: I174d8a8a22fbd8814ede25002cb2705588912329
Reviewed-on: https://go-review.googlesource.com/c/go/+/278474


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>