[x86] add and use fast horizontal vector math subtarget feature

This is the planned follow-up to D52997. Here we are reducing horizontal vector math codegen 
by default. AMD Jaguar (btver2) should have no difference with this patch because it has 
fast-hops. (If we want to set that bit for other CPUs, let me know.)

The code changes are small, but there are many test diffs. For files that are specifically 
testing for hops, I added RUNs to distinguish fast/slow, so we can see the consequences 
side-by-side. For files that are primarily concerned with codegen other than hops, I just 
updated the CHECK lines to reflect the new default codegen.

To recap the recent horizontal op story:

1. Before rL343727, we were producing hops for all subtargets for a variety of patterns. 
   Hops were likely not optimal for all targets though.
2. The IR improvement in r343727 exposed a hole in the backend hop pattern matching, so 
   we reduced hop codegen for all subtargets. That was bad for Jaguar (PR39195).
3. We restored the hop codegen for all targets with rL344141. Good for Jaguar, but 
   probably bad for other CPUs.
4. This patch allows us to distinguish when we want to produce hops, so everyone can be 
   happy. I'm not sure if we have the best predicate here, but the intent is to undo the 
   extra hop-iness that was enabled by r344141.

Differential Revision: https://reviews.llvm.org/D53095

llvm-svn: 344361