Fix multireg promotion on arm64

src/coreclr/jit/lower.cpp

@@ -7483,7 +7483,28 @@ bool Lowering::CheckMultiRegLclVar(GenTreeLclVar* lclNode, int registerCount)
         {
             if (registerCount == varDsc->lvFieldCnt)
             {
+                int hfaFields           = 0;
                 canEnregisterAsMultiReg = true;
+                for (unsigned i = 0; i < varDsc->lvFieldCnt; ++i)
+                {
+                    LclVarDsc* fieldDesc = comp->lvaGetDesc(varDsc->lvFieldLclStart + i);
+                    if (fieldDesc->lvIsHfa())
+                    {
+                        // Let's see if all fields are HFAs
+                        hfaFields++;
+                    }
+                    else if (((fieldDesc->lvFldOffset % REGSIZE_BYTES) != 0) || (fieldDesc->lvSize() >= REGSIZE_BYTES))
+                    {
+                        // Otherwise, we expect two ints/longs
+                        canEnregisterAsMultiReg = false;
+                        break;
+                    }
+                }
+                if (hfaFields != registerCount)
+                {
+                    // A mix of HFAs with non-HFAs
+                    canEnregisterAsMultiReg = false;
+                }
             }
         }
     }

src/tests/JIT/Regression/JitBlue/Runtime_91214/Runtime_91214.cs

@@ -0,0 +1,40 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.aa
+
+// Found by Antigen
+
+using System.Runtime.CompilerServices;
+using System.Runtime.Intrinsics;
+using System.Numerics;
+using Xunit;
+
+public class Runtime_91214
+{
+    [Fact]
+    public static void TestEntryPoint()
+    {
+        Method0();
+    }
+
+    public struct S
+    {
+        public Vector3 v3;
+        public bool b;
+    }
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    public static S Method2()
+    {
+        return default;
+    }
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    public static void Method0()
+    {
+        S s = Method2();
+        Log(null, s.v3);
+    }
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    public static void Log(object a, object b) { }
+}

src/tests/JIT/Regression/JitBlue/Runtime_91214/Runtime_91214.csproj

@@ -0,0 +1,8 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <Optimize>True</Optimize>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="$(MSBuildProjectName).cs" />
+  </ItemGroup>
+</Project>