Preserve per-iteration captures when lowering IIFE temp vars

Switching synthesized temp declarations from let to var in invoke_iife breaks closure semantics for arrow-IIFE inlining inside loops when compress.ecma < 2015. In patterns like for (const k in obj) { out[k] = ((k)=>()=>k)(k) }, the extracted temp now becomes a function-scoped var, so all generated closures share one binding and observe the last iteration’s value instead of each iteration’s value. This is a runtime behavior regression (not just formatting) and is visible in the updated 8246 fixture shape where the captured temp is now var inside the loop body.

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Avoid leaking inlined arrow params into outer function scope

Using var here also removes block scoping for synthesized parameter temporaries, so an inlined arrow-IIFE inside a nested block can unexpectedly alias an existing outer binding with the same name. For example, var value = 1; { ((value)=>value)(2); } now lowers through inline_fn_param into assignments against a function-scoped var value, mutating the outer value (returning 2) even though the original arrow parameter should have remained block-local shadowing.

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Prevent var/lexical redeclaration syntax errors after inlining

Lowering these synthesized temporaries to function-scoped var can make previously valid code fail to parse when the enclosing function already has a lexical binding with the same name. A valid case like function f(){ let x=0; { ((x)=>x)(1) } } can be rewritten with an injected var x, which conflicts with the existing let x in the same function scope and triggers Identifier 'x' has already been declared.

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