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

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

AFAICT this used to put all the PMADDUBSW and PSLLW instructions on Port0, while the older PMULLW+UNPCK/PACK is spread between Port0 and Port15

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