[llvm] [ConstantFold] Support scalable constant splats in ConstantFoldCastInstruction (PR #133207)

[Luke Lau via llvm-commits]

lukel97 wrote:

Rebased now that #132960 is landed, and I've also updated a bunch of test cases outside of InstCombine that I forgot to check, sorry!

https://github.com/llvm/llvm-project/pull/133207