Do not test the flavor for every "while" iteration.
Do not test the flavor for every "while" iteration.

	* src/runner/interpreter.cc (visit(ast::rConstWhile)): Cache the
	need to yield at the beginning of the loop.
	(MAYBE_YIELD): Remove.

aldebaran/urbi,aldebaran/urbi,aldebaran/urbi,urbiforge/urbi,urbiforge/urbi,urbiforge/urbi,urbiforge/urbi,urbiforge/urbi,aldebaran/urbi,urbiforge/urbi,urbiforge/urbi,urbiforge/urbi,aldebaran/urbi,urbiforge/urbi,aldebaran/urbi,aldebaran/urbi,aldebaran/urbi