I'm looking for an extension for Visual Studio where in debug mode it's possible to single step through the intermediate language beside C#.
I'm not looking for a solution to debug managed and unmanaged code.
What is your purpose? Is your IL generated by C# compiler or dynamically produced at run time? If the former one, you can use a trick of re-compiling your binary through
re-compile the .il file to get a new binary (and a new symbols)
ilasm .il [/exe|/dll] /debug
Now when debugging that specific assembly you will see IL code rather than C# code. You will also see a matching lines from original C# file if you select appropriate option in step 2.
For the case of dynamically generated IL, I would simply use WinDbg with SOS extension. It can dump IL and step through it, but it takes a bit to get used to.