[llvm] [X86][test] Remove useless pattern for VDPBF16PSZmb and add a test for broadcast folding (PR #80629)

[Benoit Jacob via llvm-commits]

bjacob wrote:

I'll let the maintainers do the actual review, just want to say thanks and yes, the added test does correspond to what https://github.com/llvm/llvm-project/issues/68810 is about. Thanks!

https://github.com/llvm/llvm-project/pull/80629