Revert "[LoopVectorize][AArch64] Enable ordered reductions by default for AArch64"

This reverts commit f4122398 to
investigate a crash exposed by it.

The patch breaks building the code below with `clang -O2 --target=aarch64-linux`

     int a;
     double b, c;
     void d() {
       for (; a; a++) {
         b += c;
         c = a;
       }
     }