JIT: fix double reporting of some GC frame slots

If there is a gc struct local that is dependently promoted, the struct
local may be untracked while the promoted gc fields of the struct are
tracked.

If so, the jit will double report the stack offset for the gc field,
first as an untracked slot, and then as a tracked slot.

Detect this case and report the slot as tracked only.

Closes #71005.

src/coreclr/jit/gcencode.cpp

@@ -4226,7 +4226,30 @@ void GCInfo::gcMakeRegPtrTable(
                     continue;
                 }
 
-                int offset = varDsc->GetStackOffset() + i * TARGET_POINTER_SIZE;
+                const unsigned int fieldOffset = i * TARGET_POINTER_SIZE;
+
+                if (varDsc->lvPromoted)
+                {
+                    assert(compiler->lvaGetPromotionType(varDsc) == Compiler::PROMOTION_TYPE_DEPENDENT);
+
+                    // See if this untracked dependently promoted struct has
+                    // a tracked field at this offset.
+                    //
+                    unsigned const   fieldLclNum = compiler->lvaGetFieldLocal(varDsc, fieldOffset);
+                    LclVarDsc* const fieldVarDsc = compiler->lvaGetDesc(fieldLclNum);
+                    assert(varTypeIsGC(fieldVarDsc->TypeGet()));
+
+                    if (fieldVarDsc->lvTracked)
+                    {
+                        JITDUMP("Untracked GC struct V%02 has tracked gc ref at + 0x%x (P-DEP promoted V%02u); will "
+                                "report slot as tracked",
+                                varNum, fieldOffset, fieldLclNum);
+                        continue;
+                    }
+                }
+
+                int const offset = varDsc->GetStackOffset() + fieldOffset;
+
 #if DOUBLE_ALIGN
                 // For genDoubleAlign(), locals are addressed relative to ESP and
                 // arguments are addressed relative to EBP.