pull MLton deep flattening fix

[shwestrick]

Just keeping track of this, we should pull it into MaPLe too:
MLton/mlton#627

[MatthewFluet]

Has MaPLe triggered the bug? More generally, should pull upstream MLton into MaPLe.

[shwestrick]

I haven't explicitly tested for it yet! I assume it would probably be reproducible in MaPLe.

[MatthewFluet]

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.