Loop IR node should have content-dependent type

[stephanemagnenat]

Currently loops are typed as (), which is incorrect. For example, even with no break-returning value statement, a loop should have type never if there is no break, or () if there is at least one break in it. Later, when break can return values, then the type of that value should be used.

One clean way to do that is to add loop information in the typing context, using a LoopFrame:

struct LoopFrame {
    result_ty: TypeVar,   // the α for this loop
    saw_break: bool,      // whether any break occurred
    label: Option<Name>,  // if labeled breaks are supported
}

This would also allow nicely the detection of breaks outside loops.