test: add test for CL 26831

Test nil check removal for access of PAUTOHEAP.

Change-Id: Id739a9cda7cd3ff173bdcccfedcad93ee90711ef
Reviewed-on: https://go-review.googlesource.com/27232
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

test/nilptr3.go

@@ -214,3 +214,11 @@ func p1() byte {
 	p := new([100]byte)
 	return p[5] // ERROR "removed nil check"
 }
+
+// make sure not to do nil check for access of PAUTOHEAP
+//go:noinline
+func (p *Struct) m() {}
+func c1() {
+	var x Struct
+	func() { x.m() }() // ERROR "removed nil check"
+}

test/nilptr3_ssa.go

@@ -226,3 +226,11 @@ func p1() byte {
 	p := new([100]byte)
 	return p[5] // ERROR "removed nil check"
 }
+
+// make sure not to do nil check for access of PAUTOHEAP
+//go:noinline
+func (p *Struct) m() {}
+func c1() {
+	var x Struct
+	func() { x.m() }() // ERROR "removed nil check"
+}