Pointer fixes/improvements.

Allow <, <=, >, >= comparisons of pointers
Allow explicit type-casting of pointers to and from integers
Fix bug in handling expressions of the form "int + ptr" ("ptr + int"
  was fine).
Fix a bug in TypeCastExpr where varying -> uniform typecasts
  would be allowed (leading to a crash later)

tests/ptr-int-1.ispc

@@ -0,0 +1,14 @@
+
+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;
+    RET[programIndex] = *((uniform int * varying)pi);
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 1;
+}