cmd/compile: fix compiler bug for constant equality comparison

The compiler incorrectly will error when comparing a nil pointer
interface to a nil pointer of any other type. Example:
(*int)(nil) == interface{}(nil)
Will error with "gc: illegal constant expression: *int == interface {}"

Fixes #16702

Change-Id: I1a15d651df2cfca6762b1783a28b377b2e6ff8c6
Reviewed-on: https://go-review.googlesource.com/27591
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/gc/const.go

@@ -827,6 +827,9 @@ func evconst(n *Node) {
 
 	// check for compatible general types (numeric, string, etc)
 	if wl != wr {
+		if wl == TINTER || wr == TINTER {
+			goto setfalse
+		}
 		goto illegal
 	}
 

test/const.go

@@ -123,9 +123,44 @@ func floats() {
 	assert(f == f1e3, "f == f1e3")
 }
 
+func interfaces() {
+	var (
+		nilN interface{}
+		nilI *int
+		five = 5
+
+		_ = nil == interface{}(nil)
+		_ = interface{}(nil) == nil
+	)
+	ii := func(i1 interface{}, i2 interface{}) bool { return i1 == i2 }
+	ni := func(n interface{}, i int) bool { return n == i }
+	in := func(i int, n interface{}) bool { return i == n }
+	pi := func(p *int, i interface{}) bool { return p == i }
+	ip := func(i interface{}, p *int) bool { return i == p }
+
+	assert((interface{}(nil) == interface{}(nil)) == ii(nilN, nilN),
+		"for interface{}==interface{} compiler == runtime")
+
+	assert(((*int)(nil) == interface{}(nil)) == pi(nilI, nilN),
+		"for *int==interface{} compiler == runtime")
+	assert((interface{}(nil) == (*int)(nil)) == ip(nilN, nilI),
+		"for interface{}==*int compiler == runtime")
+
+	assert((&five == interface{}(nil)) == pi(&five, nilN),
+		"for interface{}==*int compiler == runtime")
+	assert((interface{}(nil) == &five) == ip(nilN, &five),
+		"for interface{}==*int compiler == runtime")
+
+	assert((5 == interface{}(5)) == ni(five, five),
+		"for int==interface{} compiler == runtime")
+	assert((interface{}(5) == 5) == in(five, five),
+		"for interface{}==int comipiler == runtime")
+}
+
 func main() {
 	ints()
 	floats()
+	interfaces()
 
 	assert(ctrue == true, "ctrue == true")
 	assert(cfalse == false, "cfalse == false")