[llvm] [GISel] Add debug counter to force sdag fallback (PR #78257)

[Amara Emerson via llvm-commits]

aemerson wrote:

When I did the daemon based bisector a few years back I had to put the fallback trigger in the InstructionSelect pass, since it's possible to fall back naturally all the way through the pipeline. That is if you want to trigger after exactly *N* GISel functions compiled.

https://github.com/llvm/llvm-project/pull/78257