[llvm] [GISel] Convert zext nneg to sext if it is cheaper (PR #93842)

[Yingwei Zheng via llvm-commits]

dtcxzyw wrote:

> Why is the DAG doing this transform in the builder and not in the combiner?
> 
> ```
> def nneg_zext : GICombineRule<
>    (defs root:$root, build_fn_matchinfo:$matchinfo),
>    (match (G_ZEXT $root, $x, (MIFlags NonNeg)),
>    [{ return Helper.matchNonNegZext(${root}, ${matchinfo}); }]),
>    (apply [{ Helper.applyBuildFnMO(${root}, ${matchinfo}); }])>;
> ```

@topperc

https://github.com/llvm/llvm-project/pull/93842