[X86] In LowerEXTEND_VECTOR_INREG, emit a vector shuffle instead of directly using X86ISD::UNPCKL

The majority of the changes are because the rest of shuffle lowering/combining prefers to replace the undef input with the other operand. Using UNPCKL directly seemed to avoid this and just grabbed a randomish register for the undef which can create false dependencies.

llvm-svn: 346050