Treat all incomplete ifs as statements

[odersky]

Previously only a one-armed if was treated as a statement
where all parts were typed with Unit as expected type. We now
extend that treatment also to multi-branch ifs that lack
a final part.

Fixes #14914

[odersky]

The reason for the Scala 2 -> to Scala 3 change was that people found it surprising that

if ... then f.write(x)

would have type AnyVal and not Unit. This can mess up things if the statement is used as a building block for custom DSLs, say. I agree that this was a wart that needed fixing. But the fix introduced another surprise in that multi-branch ifs with
a missing final part now behaved differently. That's fixed by this PR.

[som-snytt]

Thanks! I wonder if there is a better term for one-armed or one-legged, to mean else-less. The structure can be multi-limbed. Maybe biology has a term for when an octopus loses an appendage, or when a salamander loses a tail.