Make the for ^$max { } optimization also work
Make the for ^$max { } optimization also work

This codegens the optimized loop also when either end of the range
specification is a native int *variable*.  This makes for loops like
this about 1.35x as fast (with only 10% of the call frames):

   my int $max = 1_000_000;
   for ^$max { }

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