Revert "[X86][SSE] Pull out combineToHorizontalAddSub helper from inside...

Revert "[X86][SSE] Pull out combineToHorizontalAddSub helper from inside (F)ADD/SUB combines. NFCI."

This reverts commit b95a1038.
Makes clang assert very early in a Chromium build. See
https://bugs.chromium.org/p/chromium/issues/detail?id=1209490#c1
for a standalone repro.