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

[Vikram Hegde via cfe-commits]

vikramRH wrote:

> In general I think it should be a fatal error when `buildCodeGenPipeline` fails.

Would a hook similar to "isGlobalISelAbortEnabled()" be an acceptable design here ? I'm looking for ways to smooth out transition. Current state (atleast for AMDGPU) is that GlobalIsel passes remain to be ported which I don't think should block the transition since we don't use that as default at least now. 
If this is not acceptable at all, the alternative as I mentioned would be to hoist out the "ISel selector" code to a more global scope and use that to decide per target if we want to enable NPM.

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