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

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

Thanks - I'm going to get the constant comment code done first, then get this patch committed, then I'll raise a new PR for non-constant multiplies.

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