[llvm] Add a pass to collect dropped var stats for MIR (PR #120780)

[Shubham Sandeep Rastogi via llvm-commits]

rastogishubham wrote:

@nikic I am very sorry about not addressing the patch issues, I will make sure to not include so much code into MachineFunctionPass.h. I had applied some aggressive filtering in my email and didn't see those comments, but I understand it is not an excuse. 

Do you have some recommendations on how to redesign this better to make it so that the build times are smaller? 

https://github.com/llvm/llvm-project/pull/120780