JIT: Fix purity of CORINFO_HELP_GETREFANY on win-x64 (#80292)

We mark this helper as pure, but on win-x64 the struct argument becomes
an implicit byref and the JIT does not support purity through the
"load".

Author: jakobbotsch

src/coreclr/jit/utils.cpp

@@ -1384,12 +1384,19 @@ void HelperCallProperties::init()
 
             // helpers returning addresses, these can also throw
             case CORINFO_HELP_UNBOX:
-            case CORINFO_HELP_GETREFANY:
             case CORINFO_HELP_LDELEMA_REF:
 
                 isPure = true;
                 break;
 
+            // GETREFANY is pure up to the value of the struct argument. We
+            // only support that when it is not an implicit byref.
+            case CORINFO_HELP_GETREFANY:
+#ifndef WINDOWS_AMD64_ABI
+                isPure = true;
+#endif
+                break;
+
             // helpers that return internal handle
             case CORINFO_HELP_GETCLASSFROMMETHODPARAM:
             case CORINFO_HELP_GETSYNCFROMCLASSHANDLE:

src/coreclr/jit/valuenum.cpp

@@ -10023,6 +10023,14 @@ void Compiler::fgValueNumberHelperCallFunc(GenTreeCall* call, VNFunc vnf, ValueN
         default:
         {
             assert(s_helperCallProperties.IsPure(eeGetHelperNum(call->gtCallMethHnd)));
+
+#ifdef DEBUG
+            for (CallArg& arg : call->gtArgs.Args())
+            {
+                assert(!arg.AbiInfo.PassedByRef &&
+                       "Helpers taking implicit byref arguments should not be marked as pure");
+            }
+#endif
         }
         break;
     }