Silence a warning.
From Vassil: substitute then / else (ROOT-6345).

VisitCompoundStmt correctly substitutes the child Stmts. When calling Visit()
on a Stmt, this substitution needs to be carried out by the caller: only
it knows what target type to request. This fixes the issue for if statements
which do not fall back to VisitStmt() because of the conditional being of bool
type. In principle we are still missing do / while statements, too - again due
to the conditional part.