Intrinsicify SequenceCompareTo(char)

[benaadams]

From #22187

Based on #22875 which is first commit

Resolves https://github.com/dotnet/coreclr/issues/22763

[benaadams]

@dotnet-bot test Windows_NT x64 Checked jitx86hwintrinsicnoavx
@dotnet-bot test Windows_NT x64 Checked jitx86hwintrinsicnosimd
@dotnet-bot test Windows_NT x64 Checked jitnox86hwintrinsic

@dotnet-bot test Windows_NT x86 Checked jitx86hwintrinsicnoavx
@dotnet-bot test Windows_NT x86 Checked jitx86hwintrinsicnosimd
@dotnet-bot test Windows_NT x86 Checked jitnox86hwintrinsic

@dotnet-bot test Ubuntu x64 Checked jitx86hwintrinsicnoavx
@dotnet-bot test Ubuntu x64 Checked jitx86hwintrinsicnosimd
@dotnet-bot test Ubuntu x64 Checked jitnox86hwintrinsic

[stephentoub]

Perf numbers, best and worst cases?

[stephentoub]

cc: @tannergooding, @GrabYourPitchforks

[tannergooding]

Going to wait to review this until after #22816. It looks to be many of the comments are going to be the same overall.

[benaadams]

Will follow up after #22505

[stephentoub]

@benaadams, what's your plan for this series of PRs? Should they be closed for now?

[benaadams]

Still planning on them; some urgency was lost when 3.0 moved to tell mode

[stephentoub]

Still planning on them; some urgency was lost when 3.0 moved to tell mode

Thanks. Do you plan to get to them in the next week and a half?
https://github.com/dotnet/coreclr/issues/27549

[maryamariyan]

Thank you for your contribution. As announced in dotnet/coreclr#27549 this repository will be moving to dotnet/runtime on November 13. If you would like to continue working on this PR after this date, the easiest way to move the change to dotnet/runtime is:

1. In your coreclr repository clone, create patch by running git format-patch origin
2. In your runtime repository clone, apply the patch by running git apply --directory src/coreclr <path to the patch created in step 1>

[benaadams]

Rebased ready for review /cc @tannergooding @CarolEidt @GrabYourPitchforks

[CarolEidt]

LGTM but I'd suggest additional comments

[CarolEidt]

IMO this could use some additional comments to explain how this handles non-multiples of vector size (i.e. that the last compare may overlap the previous, but it's still correct).

[benaadams]

Added. Realized I could delete the additional Sse2 block in the Avx2 when adding the comment as it can fall through to the existing Sse2 one.

[CarolEidt]

Excellent!

[maryamariyan]

Thank you for your contribution. As announced in #27549 the dotnet/runtime repository will be used going forward for changes to this code base. Closing this PR as no more changes will be accepted into master for this repository. If you’d like to continue working on this change please move it to dotnet/runtime.