swift234

Use assumeNonNegative for the array body count.

This should help to optimize the range check in isValidSubscript to a single unsigned integer comparison.
But this is not done yet, because it also needs an optimization in llvm, which I'm currently working on.

Together with the llvm change there will be an performance improvement for array benchmarks, where the
range check cannot be hoisted out of a loop. E.g. ~ +30% on QuickSort.



Swift SVN r23583