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 windows x64 checked) failed Jun 27, 2026 in 2h 29m 1s

Build coreclr Pri1 Runtime Tests Run windows x64 checked failed

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