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-diff-4.ispc

@@ -0,0 +1,12 @@
+
+export uniform int width() { return programCount; }
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    uniform float * varying b = aFOO;
+    b = 5 + b;
+    RET[programIndex] = b - aFOO;
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 5;
+}