rework how we handle the type of loops

First, we keep a `CoerceMany` now to find the LUB of all the break
expressions. Second, this `CoerceMany` is actually an
`Option<CoerceMany>`, and we store `None` for loops where "break with an
expression" is disallowed. This avoids silly duplicate errors about a
type mismatch, since the loops pass already reports an error that the
break cannot have an expression. Finally, since we now detect an invalid
break target during HIR lowering, refactor `find_loop` to be infallible.

Adjust tests as needed:

- some spans from breaks are slightly different
- break up a single loop into multiple since `CoerceMany` silences
  redundant and derived errors
- add a ui test that we only give on error for loop-break-value

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