[x86] allow single source horizontal op matching (PR39195)

This is intended to restore horizontal codegen to what it looked like before IR demanded elements improved in:
rL343727

As noted in PR39195:
https://bugs.llvm.org/show_bug.cgi?id=39195
...horizontal ops can be worse for performance than a shuffle+regular binop, so I've added a TODO. Ideally, we'd 
solve that in a machine instruction pass, but a quicker solution will be adding a 'HasFastHorizontalOp' feature
bit to deal with it here in the DAG.

Differential Revision: https://reviews.llvm.org/D52997

llvm-svn: 344141