Replace Create(ulong) with CreateScalar(uint) in PopCount (#37912)

* Replace Create(ulong) with CreateScalar(uint)
* Add CreateScalar() in shim

Author: kunalspathak

src/libraries/System.Private.CoreLib/src/System/Numerics/BitOperations.cs

@@ -241,11 +241,7 @@ public static int PopCount(uint value)
             {
                 // PopCount works on vector so convert input value to vector first.
 
-                // Vector64.CreateScalar(uint) generates suboptimal code by storing and
-                // loading the result to memory.
-                // See https://github.com/dotnet/runtime/issues/35976 for details.
-                // Hence use Vector64.Create(ulong) to create Vector64<ulong> and operate on that.
-                Vector64<ulong> input = Vector64.Create((ulong)value);
+                Vector64<uint> input = Vector64.CreateScalar(value);
                 Vector64<byte> aggregated = AdvSimd.Arm64.AddAcross(AdvSimd.PopCount(input.AsByte()));
                 return aggregated.ToScalar();
             }

src/libraries/System.Utf8String.Experimental/src/System/Runtime/Intrinsics/Intrinsics.Shims.cs

@@ -7,6 +7,7 @@ namespace System.Runtime.Intrinsics
     internal static class Vector64
     {
         public static Vector64<ulong> Create(ulong value) => throw new PlatformNotSupportedException();
+        public static Vector64<uint> CreateScalar(uint value) => throw new PlatformNotSupportedException();
         public static Vector64<byte> AsByte<T>(this Vector64<T> vector) where T : struct => throw new PlatformNotSupportedException();
     }
     internal readonly struct Vector64<T>