Fixed issue with aliasing local variables

ISPC++ now produces valid code, or an appropriate error message, for all
of my test cases.

tests_ispcpp/varying.ispc

@@ -0,0 +1,14 @@
+floating foo(const uniform int a, floating b) {
+    floating out = b;
+    for (int i = 1; i<a; i++) {
+        out *= b;
+    }
+
+    return out;
+}
+
+export void square(uniform int N, uniform floating b[], uniform double out[]) {
+    foreach (i = 0 ... N) {
+        out[i] = foo(2, b[i]);
+    }
+}