#1920: when considering a block starting by "while 0", the compiler optimized the

#1920: when considering a block starting by "while 0", the compiler optimized the
whole construct away, even when an 'else' clause is present::

    while 0:
        print("no")
    else:
        print("yes")

did not generate any code at all.

Now the compiler emits the 'else' block, like it already does for 'if' statements.

Backport of r60265.