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/foreach-double-1.ispc

@@ -0,0 +1,30 @@
+
+export uniform int width() { return programCount; }
+
+uniform double one = 1;
+
+void copy(uniform double dst[], uniform double src[], uniform int count) {
+    foreach (i = 0 ... count)
+        dst[i] = one * src[i];
+}
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    uniform int count = 200 + aFOO[1];
+    uniform double * uniform src = uniform new uniform double[count];
+    for (uniform int i = 0; i < count; ++i)
+        src[i] = i;
+
+    uniform double * uniform dst = uniform new uniform double[count];
+    copy(dst, src, count);
+
+    uniform int errors = 0;
+    for (uniform int i = 0; i < count; ++i)
+        if (dst[i] != src[i])
+            ++errors;
+
+    RET[programIndex] = errors; 
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 0;
+}