Fix xarch instruction size estimates#129877

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AndyAyersMS:andy/fix-x64-instruction-size-estimates

Azure Pipelines / runtime-coreclr jitstress-isas-x86 (Build coreclr Pri1 Runtime Tests Run osx x64 checked) succeeded Jun 27, 2026 in 48m 51s

Build coreclr Pri1 Runtime Tests Run osx x64 checked succeeded

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