[llvm] [LLVM][CodeGen][SVE] Refactor isel of 128-bit constant splats. (PR #185652)

[Graham Hunter via llvm-commits]

huntergr-arm wrote:

Looks ok overall.

> Doing this means we can extend coverage to include SVE operations that take an immediate operand without needing to convert more of the DAG to scalable vectors, which can potentially prevent larger NEON patterns from matching.

Does this mean you expect this PR to enable matching patterns for the immediate form which didn't before? If so, could you please add a test if it's not too convoluted to do so?

https://github.com/llvm/llvm-project/pull/185652