fix sign extension from int to long on x86_64

src/x86.cpp

@@ -895,7 +895,7 @@ moveRR(Context* c, unsigned aSize, Assembler::Register* a,
     case 4:
       if (bSize == 8) {
       	if (BytesPerWord == 8) {
-          alwaysRex(c, aSize, b, a);
+          alwaysRex(c, bSize, b, a);
           opcode(c, 0x63);
           modrm(c, 0xc0, a, b);
       	} else {

test/Longs.java

@@ -35,7 +35,13 @@ public class Longs {
     return a - (a % b);
   }
 
+  private static int negativeOne() {
+    return -1;
+  }
+
   public static void main(String[] args) {
+    expect(((long) negativeOne()) == -1);
+
     { long foo = 25214903884L;
       int radix = 10;
       expect(foo > 0);