break and continue in a for await body, and nested for await (#502)

[ASDAlexander77]

break or continue in the body of a for await, and a for await nested in another, crashed the compiler (0xC0000005, no message), under every memory model.

mlirGen(ForStatement) generated a for await body inside an async.execute region of its own, awaited once per iteration since #498. That broke two things:

• a ts.Break / ts.Continue in the body had no loop inside its region, and lowering crashed on it;
• a nested loop crashed MLIR generation itself.

The body is now generated directly in the loop, like any other loop body. The region existed only to give an await in the body an async region. But an await creates its own async.execute and waits for it wherever it appears, as in a plain for...of in an async function. So the body still runs one iteration at a time, in order (00for_await_sequential.ts, #498, still passes). The ForAwait flag no longer changes the loop's code.

New test 00for_await_control.ts covers break, continue, a nested for await, and awaits with continue and break in one body. It passes under every model; on main each of these crashed the compiler. A for await at module level runs as well.

Gate	Result
Windows full suite (Release)	3852/3852
Linux (WSL) full suite	3837/3837
DefaultLib build + its own tests.ps1 -Model, release + debug, compile + JIT	gc 159/159; rc, none 158 passed + weakref_basic skipped (gc only); all "All tests passed"
own corpus	no flips, no first-error changes (463/621; the new test passes under own)
existing async / await / generator / yield tests	146/146

Closes #502

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