[MLIR][LLVMIR][OpenMP] Changes to scan Lowering from MLIR

[anchuraj]

Scan reductions are supported in OpenMP with the the help of scan directive. Reduction clause of the for loop/simd directive takes an inscan modifier if scan reduction is specified. With an inscan modifier, the body of the directive should specify a scan directive. This PR implements the lowering logic for scan reductions in workshare loops of OpenMP.
The body of the for loop is splitted into two loops and a scan reduction loop is added in the middle.

size num_iters = <num_iters>;
<type> buffer[num_iters];
for (i: 0..<num_iters>) {
  <input phase>;
  buffer[i] = red;
}
for (int k = 0; k != ceil(log2(num_iters)); ++k) {
  i=pow(2,k)
  for (size cnt = last_iter; cnt >= i; --cnt)
    buffer[cnt] op= buffer[cnt-i];
}
for (0..<num_iters>) {
  red = buffer[i] ;
  <scan phase>;
}