Generate more efficient for regular varying 'if' statements.

For the case where we have a regular (i.e. non-'cif') 'if' statement,
the generated code just simply checks to see if any program instance
is running before running the corresponding statements.  This is a
lighter-weight check than IfStmt::emitMaskMixed() was performing.

stmt.h

@@ -126,6 +126,7 @@ private:
         active program instances want to follow just one of the 'true' or
         'false' blocks. */
     const bool doAllCheck;
+    const bool doAnyCheck;
 
     void emitMaskedTrueAndFalse(FunctionEmitContext *ctx, llvm::Value *oldMask, 
                                 llvm::Value *test) const;