JIT: Avoid removing multi-use boxes

There was an assumption that the local operand to a GT_BOX node is
single-use which was being violated when GDV clones these as arguments.
We now allow multi-uses of these locals by setting a flag when cloning
and then handle it in gtTryRemoveBoxUpstreamEffects.

There is also GTF_VAR_CLONED that would be set on the local itself, but
given that transformations can affect the operand local node arbitrarily
I went with another of these type of flags on GT_BOX instead.

Fix #72775

src/coreclr/jit/compiler.cpp

@@ -9175,6 +9175,11 @@ void cTreeFlags(Compiler* comp, GenTree* tree)
                 {
                     chars += printf("[BOX_VALUE]");
                 }
+
+                if (tree->gtFlags & GTF_BOX_CLONED)
+                {
+                    chars += printf("[BOX_CLONED]");
+                }
                 break;
 
             case GT_CNS_INT:

src/coreclr/jit/gentree.cpp

@@ -7935,6 +7935,8 @@ GenTree* Compiler::gtCloneExpr(
                 copy = new (this, GT_BOX)
                     GenTreeBox(tree->TypeGet(), tree->AsOp()->gtOp1, tree->AsBox()->gtAsgStmtWhenInlinedBoxValue,
                                tree->AsBox()->gtCopyStmtWhenInlinedBoxValue);
+                tree->AsBox()->SetCloned();
+                copy->AsBox()->SetCloned();
                 break;
 
             case GT_INTRINSIC:
@@ -13834,6 +13836,13 @@ GenTree* Compiler::gtTryRemoveBoxUpstreamEffects(GenTree* op, BoxRemovalOptions
         return nullptr;
     }
 
+    // If this box is no longer single-use, bail.
+    if (box->WasCloned())
+    {
+        JITDUMP(" bailing; unsafe to remove box that has been cloned\n");
+        return nullptr;
+    }
+
     // If we're eventually going to return the type handle, remember it now.
     GenTree* boxTypeHandle = nullptr;
     if ((options == BR_REMOVE_AND_NARROW_WANT_TYPE_HANDLE) || (options == BR_DONT_REMOVE_WANT_TYPE_HANDLE))

src/coreclr/jit/gentree.h

@@ -543,6 +543,7 @@ enum GenTreeFlags : unsigned int
     GTF_QMARK_CAST_INSTOF       = 0x80000000, // GT_QMARK -- Is this a top (not nested) level qmark created for
                                               //             castclass or instanceof?
 
+    GTF_BOX_CLONED              = 0x40000000, // GT_BOX -- this box and its operand has been cloned, cannot assume it to be single-use anymore
     GTF_BOX_VALUE               = 0x80000000, // GT_BOX -- "box" is on a value type
 
     GTF_ICON_HDL_MASK           = 0xF0000000, // Bits used by handle types below
@@ -3638,6 +3639,16 @@ struct GenTreeBox : public GenTreeUnOp
     {
     }
 #endif
+
+    bool WasCloned()
+    {
+        return (gtFlags & GTF_BOX_CLONED) != 0;
+    }
+
+    void SetCloned()
+    {
+        gtFlags |= GTF_BOX_CLONED;
+    }
 };
 
 /* gtField  -- data member ref  (GT_FIELD) */