[clang] [llvm] [openmp] [CLANG][OpenMP] Restore loop variable values after transformation directives (PR #214717)

[Alexey Bataev via cfe-commits]

alexey-bataev wrote:

Check these tests:
```
#include <stdio.h>
#include <stdlib.h>
int main() {
  int i, j, k;
  int count = 0;
#pragma omp fuse looprange(1, 2)
  {
    for (i = 0; i < 10; i++)
      ;
    for (j = 0; j < 15; j++)
      ;
    // Reads i: must observe the loop-exit value 10, not the value left by
    // the last fused iteration (9).
    for (k = 0; k < i; k++)
      count++;
  }
  printf("count=%d i=%d j=%d k=%d\n", count, i, j, k);
  return EXIT_SUCCESS;
}
```
```
#include <stdio.h>
#include <stdlib.h>
int main() {
  int i, j, l;
#pragma omp fuse
  {
#pragma omp fuse
    {
      for (i = 0; i < 10; i++)
        ;
      for (j = 0; j < 20; j++)
        ;
    }
    for (l = 0; l < 5; l++)
      ;
  }
  printf("i=%d j=%d l=%d\n", i, j, l);
  return EXIT_SUCCESS;
}
```
```

#include <stdio.h>
#include <stdlib.h>
int main() {
  int i, j, k, l;
#pragma omp fuse looprange(2, 2)
  {
#pragma omp fuse looprange(2, 2)
    {
      for (i = 0; i < 8; i++)
        ;
      for (j = 0; j < 16; j++)
        ;
      for (k = 0; k < 32; k++)
        ;
    }
    for (l = 0; l < 4; l++)
      ;
  }
  printf("i=%d j=%d k=%d l=%d\n", i, j, k, l);
  return EXIT_SUCCESS;
}
```

https://github.com/llvm/llvm-project/pull/214717