reflect: add back signaling NaN test

I was a little too agressive in CL 258957 (removing 387 support) in
removing a signaling NaN test that should probably still exist.
I should have just removed the 387 skip, not the entire test.

Change-Id: I97ca53f190eb0de00e43b96629b2cae677e7dacb
Reviewed-on: https://go-review.googlesource.com/c/go/+/348209
Trust: Keith Randall <[email protected]>
Run-TryBot: Keith Randall <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: randall77

src/reflect/all_test.go

@@ -4431,6 +4431,14 @@ var gFloat32 float32
 const snan uint32 = 0x7f800001
 
 func TestConvertNaNs(t *testing.T) {
+	// Test to see if a store followed by a load of a signaling NaN
+	// maintains the signaling bit. (This used to fail on the 387 port.)
+	gFloat32 = math.Float32frombits(snan)
+	runtime.Gosched() // make sure we don't optimize the store/load away
+	if got := math.Float32bits(gFloat32); got != snan {
+		t.Errorf("store/load of sNaN not faithful, got %x want %x", got, snan)
+	}
+	// Test reflect's conversion between float32s. See issue 36400.
 	type myFloat32 float32
 	x := V(myFloat32(math.Float32frombits(snan)))
 	y := x.Convert(TypeOf(float32(0)))