Light up `Ascii.Utility` methods with `Vector512` code paths.

[anthonycanino]

This issue tracks upgrades to Ascii.Utility with Vector512 (and in some cases Vector256) code paths.

We plan to upgrade the following methods (the ones checked off have already been merged into main).

• NarrowUtf16ToAscii
• WidenAsciiToUtf16
• GetIndexOfFirstNonAsciiChar
• GetIndexOfFirstNonAsciiByte
• IsValidCore
• Equals/EqualsIgnore
• CaseConversion

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Issue Details

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This issue tracks upgrades to Ascii.Utility with Vector512 (in in some cases Vector256) code paths.

We plan to upgrade the following methods (the ones checked off have already been merged into main).

• NarrowUtf16ToAscii
• WidenAsciiToUtf16
• GetIndexOfFirstNonAsciiChar
• GetIndexOfFirstNonAsciiByte
• IsValidCore
• Equals/EqualsIgnore
• CaseConversion

Author:	anthonycanino
Assignees:	-
Labels:	area-System.Text.Encoding, untriaged, needs-area-label
Milestone:	-

[layomia]

Triage per offline discussion with @anthonycanino:

This work isn't a must-have for 8.0, but folks still hope to get a couple PRs in soon. We'll mark as 8.0 for now; cc @BruceForstall @tannergooding who could weigh in on if/when to move this to future.