cmd/compile: do not write implicit conversion for append in Unified IR

Same as CL 418475, but for Unified IR.

Updates golang#53888
Fixes golang#54337

Change-Id: I31d5a7af04d8e3902ed25db85009d46ea4c38dbe
Reviewed-on: https://go-review.googlesource.com/c/go/+/422040
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Than McIntosh <thanm@google.com>

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

@@ -1989,6 +1989,13 @@ func (r *reader) expr() (res ir.Node) {
 		case ir.OAPPEND:
 			n := n.(*ir.CallExpr)
 			n.RType = r.rtype(pos)
+			// For append(a, b...), we don't need the implicit conversion. The typechecker already
+			// ensured that a and b are both slices with the same base type, or []byte and string.
+			if n.IsDDD {
+				if conv, ok := n.Args[1].(*ir.ConvExpr); ok && conv.Op() == ir.OCONVNOP && conv.Implicit() {
+					n.Args[1] = conv.X
+				}
+			}
 		case ir.OCOPY:
 			n := n.(*ir.BinaryExpr)
 			n.RType = r.rtype(pos)

src/cmd/compile/internal/test/issue53888_test.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build !race && !goexperiment.unified
+//go:build !race
 
 package test
 
@@ -11,7 +11,6 @@ import (
 	"testing"
 )
 
-// TODO(cuonglm,mdempsky): figure out why Unifed IR failed?
 func TestAppendOfMake(t *testing.T) {
 	testenv.SkipIfOptimizationOff(t)
 	for n := 32; n < 33; n++ { // avoid stack allocation of make()