[X86][SSE] LowerBUILD_VECTORAsVariablePermute - fix PSHUFB source/index operand ordering

As detailed in rL317463, PSHUFB (like most variable shuffle instructions) uses Op[0] for the source vector and Op[1] for the shuffle index vector, VPERMV works in reverse which is probably where the confusion comes from.

Differential Revision: https://reviews.llvm.org/D42380

llvm-svn: 323190