[TIR] Avoid re-defining var = arg_var in ArgBinder

[Lunderberg]

Prior to this commit, ArgBinder would always introduce a new variable to represent the input argument, even if the argument already a primitive type. This introduces trivial let bindings that are expected to be simplified out, but which can produce dangling tir::Var usage in some cases (see #14951).

This commit updates ArgBinder to prefer using the original tir::Var when possible. That is, when a function takes n: T.int32 as input, the packed function should produce a binding n: T.int32 = T.tvm_struct_get(...), rather than producing a binding arg_n = T.tvm_struct_get(...) followed by n = arg_n.

[tvm-bot]

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• cc @Hzfengsy, @junrushao, @quic-sanirudh, @shingjan _{See #10317 for details}

_{Generated by tvm-bot}

[Lunderberg]

@tvm-bot rerun