We duplicate ~30 lines of code to lower FABS and FNEG for x86, so I've combined them into one function. No functional change intended, so no additional test cases. Test-suite behavior is unchanged.
This is a follow-on patch to http://reviews.llvm.org/D5052 (use an integer constant for FABS / FNEG (x86)).
If this patch is OK, we can abandon the other patch because that change is included here.