Invalid iterators in LoweringPreparePass

As described in https://github.com/llvm/clangir/pull/1180#event-15638412878,

if we remove `-clangir-disable-passes` from [clang/test/CIR/CodeGen/dtor-alias.cpp](https://github.com/llvm/clangir/pull/1180/files#diff-c1b06f900c9188c39b0a24756cd960d7c7e21bbcdd880f5076e6db920c7bb7fe), then it will crash during LoweringPreparePass