Investigate potential ARM64 perf improvements in AsciiUtility and Utf16Utility

[pgovind]

This is a follow up to #39506, #39507 and #39050 to investigate Tamar's suggestions for improving ARM64 perf in ASCIIUtility and Utf16Utility. The suggestions are #39506 (comment), #39507 (comment) and #39507 (comment)

cc @carlossanlop @jeffhandley @kunalspathak @echesakovMSFT @tannergooding

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