[DAGCombine] combineRepeatedFPDivisors - recognize -1.0 / X as a reciprocal

Fixes issue identified by @nemanjai (Nemanja Ivanovic) in D62963 / rL363040 - infinite loop due to GetNegatedExpression fighting combineRepeatedFPDivisors resulting in fneg(fdiv(x,splat)) -> fneg(fmul(x,1.0/splat)) -> fmul(x,-1.0/splat) -> fmul(x,(-1.0 * 1.0)/splat) ......

llvm-svn: 364326