[llvm] [DAGCombiner][X86][WIP] Combine (build_vector (load X))->(vecty (load X)) (PR #88753)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

Just to clear - #88261 is not about improving MMX codegen (it was just a weird WTF moment). I'm happy to continue work on the riscv changes\regressions.

https://github.com/llvm/llvm-project/pull/88753