Compare i256 in validate_decimal256_precision

[viirya]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

validate_decimal256_precision validates decimal 256 values by comparing little-endian bytes of decimal value with const byte values. As we moved to i256 now, we can compare i256 directly.

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[alamb]

label_issue.py automatically added labels {'arrow'} from #3025