Temporarily Revert "[X86][SLP] Enable SLP vectorization for 128-bit horizontal...

Temporarily Revert "[X86][SLP] Enable SLP vectorization for 128-bit horizontal X86 instructions (add, sub)"

As this has broken the lto bootstrap build for 3 days and is
showing a significant regression on the Dither_benchmark results (from
the LLVM benchmark suite) -- specifically, on the
BENCHMARK_FLOYD_DITHER_128, BENCHMARK_FLOYD_DITHER_256, and
BENCHMARK_FLOYD_DITHER_512; the others are unchanged.  These have
regressed by about 28% on Skylake, 34% on Haswell, and over 40% on
Sandybridge.

This reverts commit r353923.

llvm-svn: 354434