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.

[ericstj]

Moving this to 9.0 as it's clearly too late to do more here. @BruceForstall feel free to make more changes as appropriate.

[Ruihan-Yin]

For the remaining ones:

1. Equals/EqualsIgnore: can be checked off as Light up Ascii.Equality.Equals and Ascii.Equality.EqualsIgnoreCase with Vector512 code path #88650 has been merged to main.
2. CaseConversion: Some in-place methods were not scaling well on Vector512 path compared with Vector256 due to the penalty from split vector load, which might be solved by Use StoreAligned not Store in WidenAsciiToUtf16 #89892, this PR is currently paused and targeting on .Net 9.