adding cases of 'cast' instructions in optimizations

tests/ptr-int-1.ispc

@@ -5,7 +5,7 @@ export uniform int width() { return programCount; }
 export void f_f(uniform float RET[], uniform float aFOO[]) {
     uniform int a = 1;
     uniform int * uniform b = &a;
-    int64 pi = (int64)b;
+    uintptr_t pi = (uintptr_t)b;
     RET[programIndex] = *((uniform int * varying)pi);
 }