Short-circuit evaluation of ? : operator for varying tests.

? : now short-circuits evaluation of the expressions following
the boolean test for varying test types.  (It already did this
for uniform tests).

Issue #169.

tests/short-circuit-select-2.ispc

@@ -0,0 +1,21 @@
+
+export uniform int width() { return programCount; }
+
+uniform int * uniform ptr;
+
+bool crashEven() {
+    return (programIndex & 1) ? true : (*ptr > 0);
+}
+
+export void f_fu(uniform float RET[], uniform float aFOO[], uniform float b) {
+    float a = aFOO[programIndex]; 
+    float a0 = aFOO[0], a1 = aFOO[1];
+    if (((programIndex & 1) == 1) && crashEven())
+        RET[programIndex] = 1;
+    else
+        RET[programIndex] = 2;
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = (programIndex & 1) ? 1 : 2;
+}