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_errors/func-export-task.ispc

@@ -0,0 +1,4 @@
+// Function can't have both "task" and "export" qualifiers 
+
+export task void foo() {
+}

tests_errors/ptrcast-lose-info.ispc

@@ -0,0 +1,6 @@
+// Pointer type cast of type "int32 * uniform" to integer type "uniform int32" may lose information.
+
+int32 foo(int * uniform x)  {
+    return (int32) x;
+}
+

tests_errors/vary-to-unif-typecast.ispc

@@ -0,0 +1,6 @@
+// Can't type cast from varying type "int32" to uniform type "uniform int32"
+
+uniform int foo(int x)  {
+    return (uniform int) x;
+}
+