Normally, after compiler optimizations we get textual LLVM IR, so that we can compare the IR's before and after optimizations and reason about. In LTO, we usually input IR bitcode files to the linker (lld) and get native object files. Is there any way to get single monolithic LLVM IR (textual representation) after LTO passes? And Is there any effective way to analyze the object code to find out the optimizations other than just seeing text section of object file.
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LTO optimizations are more or less the same that gets applied to the code during normal compilation. The difference is that the module being optimized comes from linking all the modules of a program.
So, you can just compile all your source to LLVM IR (with -flto, for instance), then link object files (which actually are bitcode files) using llvm-link and then play with optimizations running opt on this resulting bitcode. The list of passes applied during LTO stage can be seen in lib/Transforms/IPO/PassManagerBuilder.cpp, populateLTOPassManager(). There is also a nice opt option called -print-after to emit textual IR representation after applied given pass.