[PATCH] fix for PR20354 - Miscompile of fabs due to vectorization

[Sanjay Patel]

Hi Pete -
We're getting a miscompilation from the existing / unchanged code for visitFABS. I've tried to combine a bug fix and two optimizations into this single patch, and I think that was a mistake because that's causing confusion. 

Let me abandon this patch, submit a new patch that *only* fixes the bug for FABS, follow that up with a patch that optimizes the vector case for FABS, and finally submit a final patch that optimizes the vector case for FNEG.

http://reviews.llvm.org/D4633