swift234

enhance the constant folding pass a bit to be smarter about folding
tuple-producing results (like those produced by folding overflow builtins).

Before the pass would just RAUW the apply_inst with a tuple_inst, but this
would leave around a bunch of tuple extracts.  Now we seek and destroy them.

This unblocks other transformations and allows the stdlib to shrink by another
1700 LOC.


Swift SVN r9900