[llvm] [TableGen] Add test showing incorrect SubRegIndex for a subreg of a subreg (PR #142810)

[Ulrich Weigand via llvm-commits]

uweigand wrote:

> These definitions make no sense to me, since lh32 should be identical to h32 and ll32 should be identical to l32; whereas hl32 and hh32 are different, and should have real definitions.

This is what we had originally, but then changed in https://github.com/llvm/llvm-project/commit/e7c1b4b64c314c9adb39857d92a660557217cada to fix https://github.com/llvm/llvm-project/issues/61390

Has anything changed in the meantime so that this workaround is no longer necessary?

https://github.com/llvm/llvm-project/pull/142810