[X86] In LowerHorizontalByteSum, emit vector_shuffle nodes instead of directly using X86ISD::UNPCKL/X86ISD::UNPCKH.

This gives shuffle lowering the freedom to use zero_extend_vector_inreg for the unpckl shuffle. Shuffle combining usually makes this swap later, but not when AVX512 is enabled it seems.

While there also use DAG.getConstant to create a 0 vector instead of using the helper the forces a specific BUILD_VECTOR. I don't think that helper is usually needed. We're basically free to create a constant build_vector anytime and it will be legalized on its own.

llvm-svn: 346574

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