Do not always request an implicit taskgroup region inside the kmpc_taskloop function

Summary:
For the following code:
```
    int i;
    #pragma omp taskloop
    for (i = 0; i < 100; ++i)
    {}

    #pragma omp taskloop nogroup
    for (i = 0; i < 100; ++i)
    {}
```

Clang emits the following LLVM IR:

```
 ...
  call void @__kmpc_taskgroup(%struct.ident_t* @0, i32 %0)
  %2 = call i8* @__kmpc_omp_task_alloc(%struct.ident_t* @0, i32 %0, i32 1, i64 80, i64 8, i32 (i32, i8*)* bitcast (i32 (i32, %struct.kmp_task_t_with_privates*)* @.omp_task_entry. to i32 (i32, i8*)*))
  ...
  call void @__kmpc_taskloop(%struct.ident_t* @0, i32 %0, i8* %2, i32 1, i64* %8, i64* %9, i64 %13, i32 0, i32 0, i64 0, i8* null)
  call void @__kmpc_end_taskgroup(%struct.ident_t* @0, i32 %0)

  ...
  %15 = call i8* @__kmpc_omp_task_alloc(%struct.ident_t* @0, i32 %0, i32 1, i64 80, i64 8, i32 (i32, i8*)* bitcast (i32 (i32, %struct.kmp_task_t_with_privates.1*)* @.omp_task_entry..2 to i32 (i32, i8*)*))
  ...
  call void @__kmpc_taskloop(%struct.ident_t* @0, i32 %0, i8* %15, i32 1, i64* %21, i64* %22, i64 %26, i32 0, i32 0, i64 0, i8* null)

```

The first set of instructions corresponds to the first taskloop construct. It is important to note that the implicit taskgroup region associated with the taskloop construct has been materialized in our IR:  the `__kmpc_taskloop` occurs inside a taskgroup region. Note also that this taskgroup region does not exist in our second taskloop because we are using the `nogroup` clause.

The issue here is the 4th argument of the kmpc_taskloop call, starting from the end,  is always a zero. Checking the LLVM OpenMP RT implementation, we see that this argument corresponds to the nogroup parameter:

```
void __kmpc_taskloop(ident_t *loc, int gtid, kmp_task_t *task, int if_val,
                     kmp_uint64 *lb, kmp_uint64 *ub, kmp_int64 st, int nogroup,
                     int sched, kmp_uint64 grainsize, void *task_dup);
```

So basically we always tell to the RT to do another taskgroup region. For the first taskloop, this means that we create two taskgroup regions. For the second example, it means that despite the fact we had a nogroup clause we are going to have a taskgroup region, so we unnecessary wait until all descendant tasks have been executed.

Reviewers: ABataev

Reviewed By: ABataev

Subscribers: rogfer01, cfe-commits

Differential Revision: https://reviews.llvm.org/D53636

llvm-svn: 345180