Build the yield argument handler once, not per evaluation

[lahma]

\JintYieldExpression\ called \JintExpression.Build(expression.Argument)\ inside \EvaluateInternal\ — on every yield evaluation. \Build\ only returns a cached handler when the AST node's \UserData\ was pre-populated (prepared scripts), so for ordinary execution every \yield \ reconstructed the argument's whole handler subtree per iteration: for \yield { a: i, b: i + 1 }\ that is a \JintObjectExpression\ + \ObjectProperty[]\ + \ExpressionCache\ + identifier/binary handlers — every time the generator produced a value — and the handler lost all its per-node inline caches on each rebuild. The allocation census flagged this as ~51% of a generator-producing-literals workload (handler types should never appear in steady-state allocation profiles at all).

The argument handler is now built once in the constructor, like every other expression handler.

Measurements

Generator yielding object literals (100k yields, fresh engine per iteration, A/B/A/B alternating binaries): 132.2 → 52.9 MB/iter (−60%), 98.4 → 67.9 ms/iter (−31%). All AST-handler types disappear from the allocation profile; what remains is the literal's dictionary-path descriptors and the user-visible generator result objects.

Verification

• \Jint.Tests\ green (net10.0 + net472), \Jint.Tests.PublicInterface\ green
• Full test262: 99,260 passed, 0 failed (generator/async-generator suites cover yield, yield*, resume-with-throw/return and yields in loops)

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