[libomptarget][devicertl] Remove branches around setting parallelLevel

Simplifies control flow to allow store/load forwarding

This change folds two basic blocks into one, leaving a single store to parallelLevel.
This is a step towards spmd kernels with sufficiently aggressive inlining folding
the loads from parallelLevel and thus discarding the nested parallel handling
when it is unused.

Transform:
```
int threadId = GetThreadIdInBlock();
if (threadId == 0) {
  parallelLevel[0] = expr;
} else if (GetLaneId() == 0) {
  parallelLevel[GetWarpId()] = expr;
}
// =>
if (GetLaneId() == 0) {
  parallelLevel[GetWarpId()] = expr;
}
// because
unsigned GetLaneId() { return GetThreadIdInBlock() & (WARPSIZE - 1);}
// so whenever threadId == 0, GetLaneId() is also 0.
```

That replaces a store in two distinct basic blocks with as single store.

A more aggressive follow up is possible if the threads in the warp/wave
race to write the same value to the same address. This is not done as
part of this change.

```
if (GetLaneId() == 0) {
  parallelLevel[GetWarpId()] = expr;
}
// =>
parallelLevel[GetWarpId()] = expr;
// because
unsigned GetWarpId() { return GetThreadIdInBlock() / WARPSIZE; }
// so GetWarpId will index the same element for every thread in the warp
// and, because expr is lane-invariant in this case, every lane stores the
// same value to this unique address
```

Reviewed By: tianshilei1992

Differential Revision: https://reviews.llvm.org/D105699