Fix bugs where typecasting an expression to void would cause it to disappear.

This was obviously problematic in cases where the expression was a function
call or the like, with side effects.

Fixes issue #199.

tests/typecast-void-funcall.ispc

@@ -0,0 +1,17 @@
+
+export uniform int width() { return programCount; }
+
+uniform float add(float a, float b, uniform float * uniform result) {
+    result[programIndex] = a+b;
+    return 1;
+}
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    float a = aFOO[programIndex]; 
+    float b = 0.; b = a; 
+    (void)add(a, b, RET);
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 2 + 2*programIndex;
+}