cmd/compile: support reading union type for compiler backend in unified IR

Fixes #52124

Change-Id: I5749822d41d8e51f476bceb277b1d2cf7350dcc3
Reviewed-on: https://go-review.googlesource.com/c/go/+/397874
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

Author: cuonglm

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

@@ -442,9 +442,21 @@ func (r *reader) doTyp() *types.Type {
 		return r.structType()
 	case pkgbits.TypeInterface:
 		return r.interfaceType()
+	case pkgbits.TypeUnion:
+		return r.unionType()
 	}
 }
 
+func (r *reader) unionType() *types.Type {
+	terms := make([]*types.Type, r.Len())
+	tildes := make([]bool, len(terms))
+	for i := range terms {
+		tildes[i] = r.Bool()
+		terms[i] = r.typ()
+	}
+	return types.NewUnion(terms, tildes)
+}
+
 func (r *reader) interfaceType() *types.Type {
 	tpkg := types.LocalPkg // TODO(mdempsky): Remove after iexport is gone.
 

test/typeparam/issue52124.go

@@ -0,0 +1,9 @@
+// compile
+
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type I interface{ any | int }