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.cpp

@@ -0,0 +1,27 @@
+#include <stdlib.h>
+#include <stdio.h>
+
+#include "varying.h"
+
+int main() {
+    float A[256];
+    double B[256];
+    double outA[256];
+    double outB[256];
+
+
+    for (int i=0; i<256; i++) {
+        A[i] = 1. / (i+1);
+        B[i] = 1. / (i+1);
+    }
+
+    ispc::square(256, (float*)&A, (double*)&outA);
+
+    ispc::square(256, (double*)&B, (double*)&outB);
+
+    for (int i=0; i<256; i++) {
+        printf("float: %.16f\tdouble: %.16f\n", outA[i], outB[i]);
+    }
+
+    return 0;
+}