InstCombine: fsub nsz 0, X ==> fsub nsz -0.0, X

InstCombine: fsub nsz 0, X ==> fsub nsz -0.0, X

Some day the backend may handle instruction-level fast math flags and make
this transform unnecessary, but it's still better practice to use the canonical
representation of fneg when possible (use a -0.0).

This is a partial fix for PR20870 ( http://llvm.org/bugs/show_bug.cgi?id=20870 ).
See also http://reviews.llvm.org/D6723.

Differential Revision: http://reviews.llvm.org/D6731

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@225050 91177308-0d34-0410-b5e6-96231b3b80d8