JIT: fix spill logic for local structs

If we're appending an assignment whose LHS is is a location within a local
struct, we need to spill all references to that struct from the eval stack.

Update the existing logic for this to handle the case where the LHS is a field
of a local struct, and the field is updated by unusual means (here, `initobj`).

Fixes dotnet#764.

src/coreclr/src/jit/importer.cpp

@@ -10831,11 +10831,22 @@ void Compiler::impImportBlockCode(BasicBlock* block)
                     }
                     else if (lhs->OperIsBlk())
                     {
-                        // Check for ADDR(LCL_VAR), or ADD(ADDR(LCL_VAR),CNS_INT))
-                        // (the latter may appear explicitly in the IL).
-                        // Local field stores will cause the stack to be spilled when
-                        // they are encountered.
-                        lclVar = lhs->AsBlk()->Addr()->IsLocalAddrExpr();
+                        // Check if LHS address is within some struct local, to catch
+                        // cases where we're updating the struct by something other than a stfld
+                        GenTree* addr = lhs->AsBlk()->Addr();
+
+                        // Catches ADDR(LCL_VAR), or ADD(ADDR(LCL_VAR),CNS_INT))
+                        lclVar = addr->IsLocalAddrExpr();
+
+                        // Catches ADDR(FIELD(... ADDR(LCL_VAR)))
+                        if (lclVar == nullptr)
+                        {
+                            GenTree* lclTree = nullptr;
+                            if (impIsAddressInLocal(addr, &lclTree))
+                            {
+                                lclVar = lclTree->AsLclVarCommon();
+                            }
+                        }
                     }
                     if (lclVar != nullptr)
                     {

src/coreclr/tests/src/JIT/Regression/JitBlue/Runtime_764/Runtime_764.cs

@@ -0,0 +1,52 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+using System.Runtime.CompilerServices;
+
+// Regression test case for importer bug.
+// If Release is inlined into Main, the importer may unsafely re-order trees.
+
+public struct Ptr<T> where T: class
+{
+    private T _value;
+
+    public Ptr(T value)
+    {
+        _value = value;
+    }
+
+    public T Release()
+    {
+        T tmp = _value;
+        _value = null;
+        return tmp;
+    }
+}
+
+class Runtime_764
+{
+    private static int Main(string[] args)
+    {
+        Ptr<string> ptr = new Ptr<string>("Hello, world");
+
+        bool res = false;
+        while (res)
+        {
+        }
+
+        string summary = ptr.Release();
+
+        if (summary == null)
+        {
+            Console.WriteLine("FAILED");            
+            return -1;
+        }
+        else
+        {
+            Console.WriteLine("PASSED");            
+            return 100;
+        }
+    }
+}

src/coreclr/tests/src/JIT/Regression/JitBlue/Runtime_764/Runtime_764.csproj

@@ -0,0 +1,10 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <DebugType>None</DebugType>
+    <Optimize>True</Optimize>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="$(MSBuildProjectName).cs" />
+  </ItemGroup>
+</Project>