[clang] Flags for large CFG bail out (PR #170444)

[Utkarsh Saxena via cfe-commits]

https://github.com/usx95 requested changes to this pull request.

I do not think BlockSize is the right metric to track or right number to use for bailout.

Since `join` is the bottleneck, there are three things which contribute to this performance:  
\- Number of join operations is proportional to the number of blocks (not the size of the blocks).  
\- Number of join operations is proportional to the number of fixed point iterations.  
* Time complexity of a single join operation is proportional to the number of origins in a CFG.

I would bailout based on these three numbers only.

https://github.com/llvm/llvm-project/pull/170444