[SelectionDAG] Removing FABS folding from DAGCombiner

The code has bugs dealing with -0.0.

Since D44550 introduced FABS pattern folding in InstCombine, 
this patch removes the now-redundant code that causes 
https://bugs.llvm.org/show_bug.cgi?id=36600.

Patch by Mikhail Dvoretckii!

Differential Revision: https://reviews.llvm.org/D44683



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@328872 91177308-0d34-0410-b5e6-96231b3b80d8