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

[Matt Arsenault via cfe-commits]

arsenm wrote:

> 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.

This is overcomplicating ti. Just fatal error if the target hasn't fully implemented everything, there's much less value in incremental bringup here than globalisel 

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