[release/9.0-staging] Ensure Vector.Create is properly recognized as intrinsic

[github-actions]

Backport of #108945 to release/9.0-staging

/cc @tannergooding

Customer Impact

• Customer reported
• Found internally

#108929

Customers using System.Numerics.Vector<T> may see unexpected perf slowdowns on .NET 9 due to the new Vector(T scalar) APIs no longer being recognized as [Intrinsic] and therefore not being accelerated by the JIT.

Regression

• Yes
• No

Vector<T> is one of the original hardware accelerated types introduced in 2014 for .NET Framework. There have been multiple iterations over the past several releases modernizing the JIT implementation to bring it inline with the newer types exposed by the System.Runtime.Intrinsics namespace.

The regression was introduced in .NET 9 as part of one of the larger refactorings done for the System.Numerics.Vector<T> types when the Vector.Create<T>(T scalar) APIs were added for parity and the existing constructors were meant to defer to these APIs.

Testing

JIT disassembly was checked to ensure it was generated the expected output.

Risk

Low. This is a lowlevel performance oriented API. The regression is one of performance and not of correctness.

The fix is simply ensuring the existing logic is correctly hooked up to the new APIs as was intended.

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[tannergooding]

CC. @dotnet/jit-contrib, @BruceForstall for review

@JulieLeeMSFT this should be considered for backport since its a perf regression in one of the intrinsic APIs, which are expected to be accelerated, and a user reported the initial regression

[tannergooding]

CC. @JulieLeeMSFT, I think this is just pending management approval for backport now.

[jeffschwMSFT]

lgtm. we will take for consideration in 9.0.x

[tannergooding]

Approved by tactics over e-mail