[llvm] [X86] Lower vXi8 multiplies by constant using PMADDUBSW on SSSE3+ targets (PR #95403)

[Phoebe Wang via llvm-commits]

phoebewang wrote:

Oh, I forgot PSLLW. Do you take PAND/PANDN into account? I think non-constant cases need them, right?

https://github.com/llvm/llvm-project/pull/95403