[llvm] [TableGen] Add a !listflatten operator to TableGen (PR #109346)

[Rahul Joshi via llvm-commits]

jurahul wrote:

I realized after I implemented this that existing fold primitives can also implement this (and I see atleast one example in an AMDGPU td file that does this). But this is much easier to use. 

https://github.com/llvm/llvm-project/pull/109346