Fix bug when doing pointer math with varying integer offsets.

We were incorrectly trying to type convert the varying offset to a
uniform value, which in turn led to an incorrect compile-time error.

Fixes issue #201.

tests/ptr-math-variability.ispc

@@ -0,0 +1,12 @@
+
+export uniform int width() { return programCount; }
+
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    uniform float * uniform ptr = aFOO;
+    RET[programIndex] = *(ptr + programIndex) - 1;
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = programIndex;
+}