Do not vectorize loops with subtraction reductions

Since subtraction does not commute the loop vectorizer incorrectly vectorizes
reductions such as x = A[i] - x.

Disabling for now.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171537 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -1986,7 +1986,6 @@ LoopVectorizationLegality::isReductionInstr(Instruction *I,
     // possibly.
     return true;
   case Instruction::Add:
-  case Instruction::Sub:
     return Kind == IntegerAdd;
   case Instruction::Mul:
     return Kind == IntegerMult;