Improve code for uniform switches with a 'break' under varying control flow.

Previously, when we had a switch statement with a uniform switch condition
but a 'break' statement that was under varying control flow inside the
switch, we'd promote the switch condition to be varying so that the
break would work correctly.

Now, we leave the condition as uniform and are thus able to use the
more-efficient LLVM switch instruction in this case.

Issue #156.

tests/switch-14.ispc

@@ -0,0 +1,24 @@
+
+export uniform int width() { return programCount; }
+
+int switchit(int a, uniform int b) {
+    switch (b) {
+    case 5:
+        if (a & 1)
+            break;
+        return 2;
+    default:
+        return 42;
+    }
+    return 3;
+}
+
+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 & 1) ? 2 : 3;
+}