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

[Rahul Joshi via llvm-commits]

jurahul wrote:

My goal is to use this to flatten Intrinsic `TypeSig` field using this operator and eliminate the nested loop in IntrinsicEmitter. Also have some other use in mind, so hoping this will be useful in general.

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