[OPENMP50]Codegen for scan directive in simd loops.

Added codegen for scandirectives in simd loop. The codegen transforms
original code:

```
int x = 0;
 #pragma omp simd reduction(inscan, +: x)
for (..) {
  <first part>
  #pragma omp scan inclusive(x)
  <second part>
}
```
into
```
int x = 0;
for (..) {
  int x_priv = 0;
  <first part>
  x = x_priv + x;
  x_priv = x;
  <second part>
}
```
and
```
int x = 0;
 #pragma omp simd reduction(inscan, +: x)
for (..) {
  <first part>
  #pragma omp scan exclusive(x)
  <second part>
}
```
into
```
int x = 0;
for (..) {
  int x_priv = 0;
  <second part>
  int temp = x;
  x = x_priv + x;
  x_priv = temp;
  <first part>
}
```

Differential revision: https://reviews.llvm.org/D78232