Add support for "switch" statements.

Switches with both uniform and varying "switch" expressions are
supported.  Switch statements with varying expressions and very
large numbers of labels may not perform well; some issues to be
filed shortly will track opportunities for improving these.

tests/switch-2.ispc

@@ -0,0 +1,17 @@
+
+export uniform int width() { return programCount; }
+
+
+export void f_fu(uniform float RET[], uniform float aFOO[], uniform float b) {
+    int a = aFOO[programIndex]; 
+    switch (b) {
+    default:
+        RET[programIndex] = -1; 
+    case 5:
+        RET[programIndex] = 0; 
+    }
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 0;
+}