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

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

@phoebewang Do you have any thoughts on non-constants? I could add a IsPMADDUBSWSlow tuning tag for SandyBridge, but it'd be annoying if we had to set it for any of the generic x86-64-v levels as well.

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