Refactor LowerFABS and LowerFNEG into one function (x86) (NFC)

We duplicate ~30 lines of code to lower FABS and FNEG for x86, so this patch combines them into one function. 
No functional change intended, so no additional test cases. Test-suite behavior is unchanged.

Differential Revision: http://reviews.llvm.org/D5064

llvm-svn: 216942