swift234

Add a builtin assumeNonNegative.

It returns the argument and specifies that the value is not negative.
It has only an effect if the argument is a load or call.

The effect of this builtin is that for the load/call argument a positive range metadata is created in llvm ir.

I also added a public function _assumeNonNegative(x: Int) -> Int in the stdlib.
To be on the save side, I prefixed it with an underscore. But maybe it makes sense to make it available for all users.



Swift SVN r23582