Fix the alignment issue in the DenseElementsAttr buffer allocation

    Currently, we only make the initial address aligned with 64-bit address but
    allocate the buffer with the real size. This can cause issue when we extract
    the value by the `readBits` method, which needs to read the memory in the
    granularity of APINT_WORD_SIZE. In this CL, we rounded the allocation size to
    the multiplies of APINT_WORD_SIZE to fix the issue.

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PiperOrigin-RevId: 241816656