[llvm] Add flags to dump IR to a file before and after LLVM passes (PR #65179)

[Arthur Eubanks via llvm-commits]

aeubanks wrote:

What about an option that just redirects the PrintIRInstrumentation output to files instead of stderr? The files can be something like `$N-$modulename.ll` where `$N` is an int that increments.

https://github.com/llvm/llvm-project/pull/65179