GH-34335: [C++][Parquet] Optimize Decoding DELTA_LENGTH_BYTE_ARRAY

[mapleFU]

Rationale for this change

According to #34323 . DELTA_LENGTH_BYTE_ARRAY is much more slower. So do some optimizations.

What changes are included in this PR?

Some tiny changes

Are these changes tested?

No more tests included, these code are already tested

Are there any user-facing changes?

No

• Closes: [C++][Parquet] Improve the performance for Decoding DELTA_LENGTH_BYTE_ARRAY #34335

[github-actions]

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In the case of PARQUET issues on JIRA the title also supports:

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See also:

• Other pull requests
• Contribution Guidelines - How to contribute patches

[github-actions]

• Closes: [C++][Parquet] Improve the performance for Decoding DELTA_LENGTH_BYTE_ARRAY #34335

[github-actions]

⚠️ GitHub issue #34335has been automatically assigned in GitHub to PR creator.

[mapleFU]

This patch is currently for poc and fast verification. If it works and #34323 is merged, I'll making code for it better :)

[mapleFU]

for (int i = 0; i < max_values; ++i) {
buffer[i].ptr = data_ptr;
data_ptr += buffer[i].len;
}

By the way, I don't know would this code cause cache problem or prevent from optimization, causing pipeline hazard or compiler do not optimize here. I'd do some survey.