[llvm] [SelectionDAG] Reland debuginfo salvage for load/ext combines (PR #194700)

[Shubham Sandeep Rastogi via llvm-commits]

rastogishubham wrote:

I will squash this into one commit when reviewed, it is split up currently to help in ease of reviewing the changes 

https://github.com/llvm/llvm-project/pull/194700