Vectorizer: Add support for loop reductions.

For example:

  for (i=0; i<n; i++)
   sum += A[i] +  B[i] + i;



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@166351 91177308-0d34-0410-b5e6-96231b3b80d8

test/Transforms/LoopVectorize/gcc-examples.ll

@@ -202,9 +202,8 @@ define void @example8(i32 %x) nounwind uwtable ssp {
   ret void
 }
 
-; We can't vectorize because it has a reduction variable.
 ;CHECK: @example9
-;CHECK-NOT: <4 x i32>
+;CHECK: phi <4 x i32>
 ;CHECK: ret i32
 define i32 @example9() nounwind uwtable readonly ssp {
   br label %1