[X86][SSE] Add SCALAR_TO_VECTOR(PEXTRW/PEXTRB) support to faux shuffle combining

Generalized existing SCALAR_TO_VECTOR(EXTRACT_VECTOR_ELT) code to support AssertZext + PEXTRW/PEXTRB cases as well. 

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@304659 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed