cmd/compile/internal/types2: move some functions into different files (cleanup)

- move structuralType/structuralString into type.go
- move functions exported for the compiler into compilersupport.go
- updated/added comments
- removed AsNamed and AsInterface - not needed by compiler

No semantic changes.

Change-Id: Ia454a49edafd627c2a25b0b71db4aa93ddd7f1f2
Reviewed-on: https://go-review.googlesource.com/c/go/+/362995
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

src/cmd/compile/internal/noder/expr.go

@@ -266,7 +266,7 @@ func (g *irgen) selectorExpr(pos src.XPos, typ types2.Type, expr *syntax.Selecto
 			if wantPtr {
 				recvType2Base = types2.AsPointer(recvType2).Elem()
 			}
-			if types2.AsNamed(recvType2Base).TypeParams().Len() > 0 {
+			if recvType2Base.(*types2.Named).TypeParams().Len() > 0 {
 				// recvType2 is the original generic type that is
 				// instantiated for this method call.
 				// selinfo.Recv() is the instantiated type
@@ -338,7 +338,7 @@ func (g *irgen) compLit(typ types2.Type, lit *syntax.CompositeLit) ir.Node {
 		return typed(g.typ(typ), n)
 	}
 
-	_, isStruct := types2.Structure(typ).(*types2.Struct)
+	_, isStruct := types2.StructuralType(typ).(*types2.Struct)
 
 	exprs := make([]ir.Node, len(lit.ElemList))
 	for i, elem := range lit.ElemList {