[mlir][LLVMIR] Fix identified structs with same name

Different identified struct types may have the same name ("").
Previously, these were deduplicated based on their name, which caused
an assertion failure when nesting identified structs:

    %0 = type { %1 }
    %1 = type { i8 }
    declare void @fn(%0)

Reviewed By: gysit

Differential Revision: https://reviews.llvm.org/D156531