BigInt (a/d)*d + (a%d) drops the remainder term when operands are for...of bindings #9143

Found while benchmarking BigInt division (#9141); reproduces on pristine main 653e886699, unrelated to that change.

Repro

The division identity (a / d) * d + (a % d) === a evaluates to (a / d) * d — the remainder term vanishes — when a and d are bindings from a for...of over a BigInt array:

for (const a of [123456789012345678901234567890n]) {
  for (const d of [1000000007n]) {
    console.log((a / d) * d + (a % d));  // node: a   perry: (a/d)*d
  }
}

Correct when the same operands are compile-time constants or plain locals — so this is about how for...of bindings type/lower in a compound BigInt expression, not about the division itself (division and remainder each produce correct values individually).

Repro files: secret-tests/scratchpad/probe_q5.ts, probe_q6.ts.

Why it matters

Silent wrong arithmetic in the shape most real code uses to iterate big-number collections, and the dropped term makes results look plausible rather than obviously broken.