[llvm] [AArch64][SDAG] Generate TBL for shuffles in single block loops. (PR #227593)

[David Green via llvm-commits]

https://github.com/davemgreen commented:

> Applying this specifically to single-block looks feels a little weird, but I guess we need to start somewhere.

Yeah I was a little unsure about it, whether we would need to use full loop info and cache the result as oppose to using the block directly. It seems OK as a simple heuristic and we can alter it in the future as it matures.

https://github.com/llvm/llvm-project/pull/227593