Fix bug with indexing into varying pointer w/uniform index.

Issue #182.

tests/ptr-varying-unif-index.ispc

@@ -0,0 +1,14 @@
+
+export uniform int width() { return programCount; }
+
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    uniform float buf[programCount];
+    uniform float * varying ptr = &buf[programCount - 1 - programIndex];
+    ptr[0] = programIndex;
+    RET[programIndex] = buf[programIndex];
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = programCount - 1 - programIndex;
+}