LoopVectorize: Use the widest induction variable type

Use the widest induction type encountered for the cannonical induction variable.

We used to turn the following loop into an empty loop because we used i8 as
induction variable type and truncated 1024 to 0 as trip count.

int a[1024];
void fail() {
  int reverse_induction = 1023;
  unsigned char forward_induction = 0;
  while ((reverse_induction) >= 0) {
    forward_induction++;
    a[reverse_induction] = forward_induction;
    --reverse_induction;
  }
}

radar://13862901

llvm-svn: 181667