Add unittest.

tests/ptr-arith-indexing.ispc

@@ -0,0 +1,16 @@
+
+export uniform int width() { return programCount; }
+
+int foo(uniform float * uniform base, uniform int uOfs, varying int vOfs) {
+    return (base+uOfs)[vOfs];
+}
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    uniform float * uniform ptr = &aFOO[0];
+    int val = foo(ptr, programCount, programIndex);
+    RET[programIndex] = val;
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 1+programCount+programIndex;
+}