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-5.ispc

@@ -0,0 +1,22 @@
+
+export uniform int width() { return programCount; }
+
+int switchit(int a, uniform int b) {
+    int r = 0;
+    switch (a) {
+    case 3:
+        return 1;
+    default:
+        return 0;
+    }
+}
+
+export void f_fu(uniform float RET[], uniform float aFOO[], uniform float b) {
+    int a = aFOO[programIndex]; 
+    int x = switchit(a, b);
+    RET[programIndex] = x; 
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = (programIndex == 2) ? 1 : 0;
+}