Optimize VectorX<T>.ConditionalSelect for constant masks

[ezhevita]

Resolves #104001.
I’ve also realized that it is not possible to implement this for arrays as I’ve originally intended since we have no immutability guarantees, so instead I’ve made sure it gets optimized with ReadOnlySpan<T> property and static VectorX<T> field.

Example:

[DisassemblyDiagnoser]
public class TestClass
{
    private ReadOnlySpan<byte> Mask => [255, 0, 255, 0, 255, 0, 255, 0, 255, 0, 255, 0, 255, 0, 255, 0];

    [Benchmark]
    public byte Span()
    {
        return Vector128.ConditionalSelect(Vector128.Create(Mask), Vector128<byte>.Zero, Vector128<byte>.One)[0];
    }
}

Old codegen:

; Method TestNamespace.TestClass:Span():ubyte:this (FullOpts)
G_M000_IG01:

G_M000_IG02:
       vmovups  xmm0, xmmword ptr [reloc @RWD00]
       vxorps   xmm1, xmm1, xmm1
       vpand    xmm1, xmm1, xmm0
       vpandn   xmm0, xmm0, xmmword ptr [reloc @RWD16]
       vpor     xmm0, xmm0, xmm1
       vmovd    eax, xmm0
       movzx    rax, al

G_M000_IG03:
       ret      
RWD00  	dq	00FF00FF00FF00FFh, 00FF00FF00FF00FFh
RWD16  	dq	0101010101010101h, 0101010101010101h
; Total bytes of code: 36

New codegen:

; Method TestNamespace.TestClass:Span():ubyte:this (FullOpts)
G_M000_IG01:

G_M000_IG02:
       vmovups  xmm0, xmmword ptr [reloc @RWD00]
       vxorps   xmm1, xmm1, xmm1
       vmovups  xmm2, xmmword ptr [reloc @RWD16]
       vpblendvb xmm0, xmm2, xmm1, xmm0
       vmovd    eax, xmm0
       movzx    rax, al

G_M000_IG03:
       ret      
RWD00  	dq	00FF00FF00FF00FFh, 00FF00FF00FF00FFh
RWD16  	dq	0101010101010101h, 0101010101010101h
; Total bytes of code: 34

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[ezhevita]

@dotnet-policy-service agree

[tannergooding]

This comment isn't applicable to the general query, it was specific to the CndSel lowering logic

[tannergooding]

@EgorBo, this should be good for secondary sign-off and merging now

[EgorBo]

Thanks!