[InstCombine] canonicalize fcmp+select to fabs

This is complicated by -0.0 and nan. This is based on the DAG patterns 
as shown in D44091. I'm hoping that we can just remove those DAG folds 
and always rely on IR canonicalization to handle the matching to fabs.

We would still need to delete the broken code from DAGCombiner to fix 
PR36600:
https://bugs.llvm.org/show_bug.cgi?id=36600

Differential Revision: https://reviews.llvm.org/D44550

llvm-svn: 327858