[dev.typeparams] cmd/compile: fix small -G=3 issues for tests disabled in run.go

- set correct position for closure capture variable in (*irgen).use()
   (issue20250.go) Also, evaluate rhs, lhs in that order in assignment
   statements to match noder1 (affects ordering of closure variables).

 - make sure to set Assign flag properly in (*irgen).forStmt() for range
   variables which are map accesses (issue9691.go)

 - make sure CheckSize() is call on the base type for top-level types
   converted by (*irgen).typ() that are pointer types (issue20174.go and
   issue37837.go)

 - deal with parentheses properly in validation function
   (*irgen).validate() (issue17270.go)

 - avoid HasNil call on type TTYPEPARAM - types2 typechecker will have
   already checked validity of the typeparam having nil value (new test
   issue39755.go)

Change-Id: Ie68004d964698aea047e19e7dcd79b297e9d47ca
Reviewed-on: https://go-review.googlesource.com/c/go/+/334733
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -57,7 +57,10 @@ func (g *irgen) stmt(stmt syntax.Stmt) ir.Node {
 			if stmt.Rhs == nil {
 				n = IncDec(g.pos(stmt), op, g.expr(stmt.Lhs))
 			} else {
-				n = ir.NewAssignOpStmt(g.pos(stmt), op, g.expr(stmt.Lhs), g.expr(stmt.Rhs))
+				// Eval rhs before lhs, for compatibility with noder1
+				rhs := g.expr(stmt.Rhs)
+				lhs := g.expr(stmt.Lhs)
+				n = ir.NewAssignOpStmt(g.pos(stmt), op, lhs, rhs)
 			}
 			if n.X.Typecheck() == 3 {
 				n.SetTypecheck(3)
@@ -68,8 +71,9 @@ func (g *irgen) stmt(stmt syntax.Stmt) ir.Node {
 			return n
 		}
 
-		names, lhs := g.assignList(stmt.Lhs, stmt.Op == syntax.Def)
+		// Eval rhs before lhs, for compatibility with noder1
 		rhs := g.exprList(stmt.Rhs)
+		names, lhs := g.assignList(stmt.Lhs, stmt.Op == syntax.Def)
 
 		// We must delay transforming the assign statement if any of the
 		// lhs or rhs nodes are also delayed, since transformAssign needs
@@ -262,6 +266,12 @@ func (g *irgen) forStmt(stmt *syntax.ForStmt) ir.Node {
 		key, value := unpackTwo(lhs)
 		n := ir.NewRangeStmt(g.pos(r), key, value, g.expr(r.X), g.blockStmt(stmt.Body))
 		n.Def = initDefn(n, names)
+		if key != nil {
+			transformCheckAssign(n, key)
+		}
+		if value != nil {
+			transformCheckAssign(n, value)
+		}
 		return n
 	}