[llvm] [Xtensa] Initial codegen support from IR (PR #78548)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> @MaskRay sorry for bothering you. I am from Espressif LLVM team.
> 
> > I think the patch series https://reviews.llvm.org/D145658 may be too fine-grained. It might be better to pack more changes in one PR, otherwise there could be a lot of `-Wunused-private-field` and not many testable units.
> 
> Looks like this conflicts with suggestions from @s-barannikov [#78548 (review)](https://github.com/llvm/llvm-project/pull/78548#pullrequestreview-1837076213). As far as I understood he proposed to split it into more fine-grained commits. Or I miss smth?

The split is better to review, but the pieces should build warning free and come with the relevant tests. I think this patch is fine with the warnings fixed 

https://github.com/llvm/llvm-project/pull/78548