[PATCH] optimize away fabs calls when input is known positive

[Sanjay Patel]

Hi hfinkel, beanz, resistor,

This patch eliminates library calls and intrinsic calls to fabs when the input is a squared value. 
This pattern will occur when a subsequent patch that I am working on to optimize sqrt calls is introduced, so I thought it'd be better to send this one out for review first because it's smaller.
Note that no unsafe-math / fast-math assumptions are needed for this optimization.

http://reviews.llvm.org/D5777

Files:
  include/llvm/Transforms/Utils/SimplifyLibCalls.h
  lib/Transforms/Utils/SimplifyLibCalls.cpp
  test/Transforms/InstCombine/fabs.ll
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