[TTI CostModel] change default cost of FP ops to 1 (PR36280)

This change was mentioned at least as far back as:
https://bugs.llvm.org/show_bug.cgi?id=26837#c26
...and I found a real program that is harmed by this: 
Himeno running on AMD Jaguar gets 6% slower with SLP vectorization:
https://bugs.llvm.org/show_bug.cgi?id=36280
...but the change here appears to solve that bug only accidentally.

The div/rem costs for x86 look very wrong in some cases, but that's already true, 
so we can fix those in follow-up patches. There's also evidence that more cost model
changes are needed to solve SLP problems as shown in D42981, but that's an independent 
problem (though the solution may be adjusted after this change is made).

Differential Revision: https://reviews.llvm.org/D43079

llvm-svn: 325515