pull MLton deep flattening fix
Just keeping track of this, we should pull it into MaPLe too: https://github.com/MLton/mlton/pull/627
Has MaPLe triggered the bug? More generally, should pull upstream MLton into MaPLe.
I haven't explicitly tested for it yet! I assume it would probably be reproducible in MaPLe.
The interesting thing about that bug is that we didn't see in really "in the wild". It came up in the context of a MS capstone project that is modifying MLton and in the absence of some forthcoming optimizations generates IR-code that is likely inhibiting tail-calls, which don't get turned into loops, which inhibits inlining; so the resulting IR program at deepFlatten is pretty different from what iis typical.