Arm64 SVE: Assert failure when TrueMask used in conditional select

[a74nh]

    static unsafe void CndSelectEmbeddedOp3LoadTrueMask(int[] array, Vector<int> op1)
    {
        fixed (int* arr_ptr = array)
        {
            var result = Sve.ConditionalSelect(Sve.CreateTrueMaskInt32(), op1, Sve.LoadVectorNonFaulting(arr_ptr));
            Consume(result);
        }
    }

Assert failure(PID 1322427 [0x00142dbb], Thread: 1322427 [0x142dbb]): Assertion failed '!node->IsEmbMaskOp()' in 'EmbeddedLoads:CndSelectEmbeddedOp3LoadTrueMask(int[],System.Numerics.Vector`1[int])' during 'Generate code' (IL size 51; hash 0xedf2dbdb; FullOpts)

    File: /mnt/sdb/home/alahay01/dotnet/runtime_table/src/coreclr/jit/hwintrinsiccodegenarm64.cpp:1093
    Image: /mnt/sdb/home/alahay01/dotnet/runtime_table/artifacts/tests/coreclr/linux.arm64.Checked/Tests/Core_Root/corerun

This is because LIR optimises ConditionalSelect(True, x, EmbeddedConditionalSelect(True, Load(), 0)) to x. Later the EmbeddedConditionalSelect() is removed as unused. However, the Load() is kept because it's a load/store.
The Load() fails to codegen becuase it is not inside an EmbeddedConditionalSelect()

There are similar tests in HWIntrinsics, but they use Vector<int>.AllBitsSet instead of truemask, and so the optimisation in lowering never happens

[a74nh]

Fix incoming....

[dotnet-policy-service]

Tagging subscribers to this area: @JulieLeeMSFT, @jakobbotsch
See info in area-owners.md if you want to be subscribed.

[a74nh]

I've been thinking about this. It may make sense that instead of adding a flag we instead introduce a new node EmbeddingConditionalSelect.

1. It reduces the number of flags (which are in short supply)
2. It's obvious when debugging (code and dump files) which conditionalselect is the embedding one. Currently I find it confusing when reading a dump file.
3. EmbeddingConditionalSelect should never use the same optimisations as a normal conditionalSelect.
4. It matches AMD64 implementation