[interp] Fix vtype ldfld from another vtype (#8496)

[interp] Fix vtype ldfld from another vtype (#8496)

CEE_LDFLD replaces at the top of the stack the object/valuetype one of its fields. MINT_LDFLD_VT loads a valuetype field from the object/valuetype at the top of the stack. When loading a field from a valuetype, at the end we also did a MINT_VTRESULT to pop the vtspace. If previously we were loading also a valuetype, then, when popping the original valuetype, we discard memory from the loaded field. With this change, we use 2 MINT_VTRESULT, one to pop the loaded field and second to copy it in place of the original valuetype.