fgVNBasedIntrinsicExpansionForCall_ReadUtf8 supports only ASCII

[stephentoub]

We should be able to lift this limitation.
cc: @EgorBo

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Issue Details

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We should be able to lift this limitation.
cc: @EgorBo

Author:	stephentoub
Assignees:	-
Labels:	area-CodeGen-coreclr, untriaged
Milestone:	-