Fix a bug in fast-math fadd/fsub simplification. 

The problem is that the code mistakenly took for granted that following constructor 
is able to create an APFloat from a *SIGNED* integer:
   
  APFloat::APFloat(const fltSemantics &ourSemantics, integerPart value)

rdar://13486998


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177906 91177308-0d34-0410-b5e6-96231b3b80d8