Add support for enums.

tests/enum-4.ispc

@@ -0,0 +1,26 @@
+
+enum Foo {
+    ZERO,
+    ONE,
+    TWO,
+    TEN = 10,
+    ELEVEN
+};
+
+enum Bar {
+    FIFTY = 50,
+    THOUSAND = 1000
+};
+
+export uniform int width() { return programCount; }
+
+export void f_fu(uniform float RET[], uniform float aFOO[], uniform float b) {
+    float a = aFOO[programIndex];
+    Foo foo = (Foo)((a == 1.) ? ELEVEN : FIFTY);
+    RET[programIndex] = (int)foo; 
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 50;
+    RET[0] = 11;
+}