[llvm] [LLVM][SelectionDAG] Don't legalise splat constants until required. (PR #143571)

[Paul Walker via llvm-commits]

paulwalker-arm wrote:

After some digging I found that each "fix" triggered other test changes.  Nothing insurmountable but certainly diverging from my original goal of having a minimal change that worked well for current and new vector constant forms.  For this reason I've followed @preames advice and created https://github.com/llvm/llvm-project/pull/144395.

https://github.com/llvm/llvm-project/pull/143571