Short-circuit evaluation of && and || operators.

We now follow C's approach of evaluating these: we don't evaluate
the second expression in the operator if the value of the first one
determines the overall result.  Thus, these can now be used 
idiomatically like (index < limit && array[index] > 0) and such.

For varying expressions, the mask is set appropriately when evaluating
the second expression.

(For expressions that can be determined to be both simple and safe to
evaluate with the mask all off, we still evaluate both sides and compute
the logical op result directly, which saves a number of branches and tests.
However, the effect of this should never be visible to the programmer.)

Issue #4.

ast.h

@@ -144,4 +144,8 @@ extern Stmt *TypeCheck(Stmt *);
     the given root. */
 extern int EstimateCost(ASTNode *root);
 
+/** Returns true if it would be safe to run the given code with an "all
+    off" mask. */ 
+extern bool SafeToRunWithMaskAllOff(ASTNode *root);
+
 #endif // ISPC_AST_H