Add lowering for As*() and dependent methods.

[DeepakRajendrakumaran]

This includes the following Vector512 API methods:
As(), AsByte(), AsDouble(), AsInt16(), AsInt32(), AsInt64(), AsNInt(), AsNUInt(), AsByte(), AsSingle(), AsUInt16(), AsUInt32(), AsUInt64(), AsVector(), AsVector512(), GetLower(), ToVector512()

Open :

1. GetUpper() is not lowered for Vector256 even though it's supported. Is this by choice?
2. I've marked this in code as a comment too : https://github.com/dotnet/runtime/blob/main/src/coreclr/jit/hwintrinsicxarch.cpp#L826-L841 : For us to get here Vector256.AsVector() or Vector256.AsVector256() has to be lowered which will happen only when target supports AVX2. Considering this, will we ever end up with a case where Vector<T> size = 128 and Vector256 gets lowered? I've not been able to manipulate .NET to hit this and this might be dead code. This extends for Vector512.AsVector() or Vector512.AsVector512() with xmm too

@dotnet/avx512-contrib

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Issue Details

---

This includes the following:
As(), AsByte(), AsDouble(), AsInt16(), AsInt32(), AsInt64(), AsNInt(), AsNUInt(), AsByte(), AsSingle(), AsUInt16(), AsUInt32(), AsUInt64(), AsVector(), AsVector512(), GetLower(), ToVector512()

Author:	DeepakRajendrakumaran
Assignees:	-
Labels:	area-CodeGen-coreclr, community-contribution
Milestone:	-

[tannergooding]

GetUpper() is not lowered for Vector256 even though it's supported. Is this by choice?

Not really, its on the backlog to add direct intrinsification and a general gtNewSimdGetUpperNode helper.

I've marked this in code as a comment too : https://github.com/dotnet/runtime/blob/main/src/coreclr/jit/hwintrinsicxarch.cpp#L826-L841 : For us to get here Vector256.AsVector() or Vector256.AsVector256() has to be lowered which will happen only when target supports AVX2. Considering this, will we ever end up with a case where Vector size = 128 and Vector256 gets lowered? I've not been able to manipulate .NET to hit this and this might be dead code. This extends for Vector512.AsVector() or Vector512.AsVector512() with xmm too

This is hit when Avx is supported but Avx2 is not supported. Try setting DOTNET_EnableAVX2=0. That should result in Vector256<T>.IsHardwareAccelerated == true but sizeof(Vector<T>) == 16.

We accelerate only a subset of Vector256 in this case, notably the ones flagged as HW_Flag_AvxOnlyCompatible

[tannergooding]

Changes overall LGTM. Just a few code cleanup requests.

[tannergooding]

Not really, its on the backlog to add direct intrinsification and a general gtNewSimdGetUpperNode helper.

Draft PR for that is here: #83982. I plan on finishing it after this PR goes in and will have it cover the new Vector512 paths you added at that time.

[tannergooding]

Failure is unrelated and being handled by #84012

[tannergooding]

CC. @dotnet/jit-contrib, @dotnet/avx512-contrib for secondary review and merge