[clang] [CodeGen] Implement fallback path for backend NPM run (PR #210248)

[Vikram Hegde via cfe-commits]

vikramRH wrote:

> I don't think there should be be a fallback. This isn't globalisel

The selector decision happens deep inside the pipeline (addCoreISelPasses()), and NPM and legacy use their own implementations. It might be possible to hoist out the code and make it common for both NPM and legacy and use that to set a target option. however I see couple of issues with that approach,
1. A single option to control all paths, i.e enabling cannot be in steps.
2. would it be proper to share code (which involves backend options) between NPM and legacy ?

https://github.com/llvm/llvm-project/pull/210248