math/big: bug in AndNot(x,y) for x>0,y<0.

The comment says to use (y-1), but then we did add(y.abs, natOne).  We meant sub.

Fixes #9609

Change-Id: I4fe4783326ca082c05588310a0af7895a48fc779
Reviewed-on: https://go-review.googlesource.com/2961Reviewed-by: Robert Griesemer <gri@golang.org>

src/math/big/int.go

@@ -890,7 +890,7 @@ func (z *Int) AndNot(x, y *Int) *Int {
 	}
 
 	// x &^ (-y) == x &^ ^(y-1) == x & (y-1)
-	y1 := nat(nil).add(y.abs, natOne)
+	y1 := nat(nil).sub(y.abs, natOne)
 	z.abs = z.abs.and(x.abs, y1)
 	z.neg = false
 	return z

src/math/big/int_test.go

@@ -1216,6 +1216,7 @@ var bitwiseTests = []struct {
 	{"-0x01", "-0x01", "-0x01", "-0x01", "0x00", "0x00"},
 	{"0x07", "0x08", "0x00", "0x0f", "0x0f", "0x07"},
 	{"0x05", "0x0f", "0x05", "0x0f", "0x0a", "0x00"},
+	{"0xff", "-0x0a", "0xf6", "-0x01", "-0xf7", "0x09"},
 	{"0x013ff6", "0x9a4e", "0x1a46", "0x01bffe", "0x01a5b8", "0x0125b0"},
 	{"-0x013ff6", "0x9a4e", "0x800a", "-0x0125b2", "-0x01a5bc", "-0x01c000"},
 	{"-0x013ff6", "-0x9a4e", "-0x01bffe", "-0x1a46", "0x01a5b8", "0x8008"},