[OPENMP50]Codegen for scan directive in simd loops.
Added codegen for scan directives 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