[X86][SSE] Add MULHS/MULHU custom lowering for i8 vectors

Currently this is to mainly to prevent scalarization of integer division by constants.

Differential Revision: http://reviews.llvm.org/D18307

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@264511 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed