use register for indexing if constant offset is too large (or too small)

Immediate indexes on ARM must be no more than 12 bits, so we must use
a temporary register for values which don't fit.

test/Arrays.java

@@ -54,5 +54,11 @@ public class Arrays {
       p = false;
       expect(array[1] == array[p ? 0 : 1]);
     }
+
+    { int[] array = new int[1024];
+      array[1023] = -1;
+      expect(array[1023] == -1);
+      expect(array[1022] == 0);
+    }
   }
 }