Array reduce typed its accumulator by the call's checked type, widened
with undefined only when the callback could return a missing value or a
receiver without an initial value could have holes. Three runtime values
fell outside it and threw "not representable in the target union":
- an initial value read from a missing element;
- a callback result of another primitive (`missing[0] ?? "fallback"`);
- a callback wrapped in parentheses, which the analysis and the callback
lowering did not see as the arrow it wraps.
The accumulator now also takes undefined from a possibly missing initial
value, and the primitive arms the callback's result can have. Array
callbacks are taken without their parentheses.
`arguments.length` in a fixed-arity function expression folded to its
declared parameter count. When the expression flows into a fixed-arity
function type with more parameters, as array callbacks do (value, index,
array, and the accumulator for reduce), every call passes that type's
count, and the length is that count.
