[TIR][BugFix]Ensure the Var's scope is correct

[sisleyli]

In the previous bf16legalize pass, the storage_scope of the original BufferVar was not retained, but all changed to the default value (global). The purpose of this PR is to ensure that Var's scope is always correct

[tvm-bot]

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

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[sisleyli]

Hi @junrushao @masahi , could you help me review this PR? Thank you!

[Hzfengsy]

Please add new testcases instead of changing the existing ones

[sisleyli]

@Hzfengsy Done. Please take another look, thank you.

[sisleyli]

@tvm-bot rerun

[sisleyli]

this PR has passed CI, could you please help me merge it? Thanks @Hzfengsy

[Hzfengsy]

Thanks @sisleyli