Handle ConstantExpressions when computing address+offset vectors for scatter/gather.

In particular, this fixes issue #81, where a global variable access was leading to
ConstantExpressions showing up in this code, which it wasn't previously expecting.

tests/global-array.ispc

@@ -0,0 +1,15 @@
+
+uniform float a[100];
+
+export uniform int width() { return programCount; }
+
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    for (uniform int i = 0; i < programCount; ++i)
+        a[i] = i;
+    RET[programIndex] = a[programIndex]; 
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = programIndex;
+}