Add foreach_unique iteration construct.

Idea via Ingo Wald / IVL compiler.

tests/foreach-unique-7.ispc

@@ -0,0 +1,13 @@
+
+export uniform int width() { return programCount; }
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    RET[programIndex] = aFOO[programIndex]; 
+
+    foreach_unique (p in programIndex & 1)
+        RET[programIndex] = p;
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = programIndex & 1;
+}