[x86] make horizontal binop matching clearer; NFCI

The instructions are complicated, so this code will
probably never be very obvious, but hopefully this
makes it better. 

As shown in PR39195:
https://bugs.llvm.org/show_bug.cgi?id=39195
...we need to improve the matching to not miss cases
where we're h-opping on 1 source vector, and that
should be a small patch after this rearranging.

llvm-svn: 343989