[OpenMP] Manually unroll the argument copy loop

[jhuber6]

Summary

The unroll pragma did not properly work as the loop bound was not known
when we optimize the runtime and we then added a "unroll disable"
metadata which prevented unrolling later when the bounds were known.
For now we manually unroll to make sure up to 16 elements are handled
nicely. This helps optimizations to look through the argument passing.

[jhuber6]

LGTM

[jhuber6]

Update for new runtime.

[jdoerfert]

LG

openmp/libomptarget/DeviceRTL/src/Parallelism.cpp

if (!IsActiveParallelRegion) {
     state::ValueRAII LevelRAII(icv::Level, 1u, 0u, true, ident);
     invokeMicrotask(TId, 0, fn, args, nargs);
     return;
   }
 
   void **GlobalArgs = nullptr;
   if (nargs) {
     __kmpc_begin_sharing_variables(&GlobalArgs, nargs);
-#pragma unroll
+    switch (nargs) {
+    default:
       for (int I = 0; I < nargs; I++)
         GlobalArgs[I] = args[I];
+      break;
+    case 16:
+      GlobalArgs[15] = args[15];
+      // FALLTHROUGH
+    case 15:
+      GlobalArgs[14] = args[14];
+      // FALLTHROUGH
+    case 14:
+      GlobalArgs[13] = args[13];
+      // FALLTHROUGH
+    case 13:
+      GlobalArgs[12] = args[12];
+      // FALLTHROUGH
+    case 12:
+      GlobalArgs[11] = args[11];
+      // FALLTHROUGH
+    case 11:
+      GlobalArgs[10] = args[10];
+      // FALLTHROUGH
+    case 10:
+      GlobalArgs[9] = args[9];
+      // FALLTHROUGH
+    case 9:
+      GlobalArgs[8] = args[8];
+      // FALLTHROUGH
+    case 8:
+      GlobalArgs[7] = args[7];
+      // FALLTHROUGH
+    case 7:
+      GlobalArgs[6] = args[6];
+      // FALLTHROUGH
+    case 6:
+      GlobalArgs[5] = args[5];
+      // FALLTHROUGH
+    case 5:
+      GlobalArgs[4] = args[4];
+      // FALLTHROUGH
+    case 4:
+      GlobalArgs[3] = args[3];
+      // FALLTHROUGH
+    case 3:
+      GlobalArgs[2] = args[2];
+      // FALLTHROUGH
+    case 2:
+      GlobalArgs[1] = args[1];
+      // FALLTHROUGH
+    case 1:
+      GlobalArgs[0] = args[0];
+      // FALLTHROUGH
+    case 0:
+      break;
+    }
   }
 
   {
     // Note that the order here is important. `icv::Level` has to be updated
     // last or the other updates will cause a thread specific state to be
     // created.
     state::ValueRAII ParallelTeamSizeRAII(state::ParallelTeamSize, NumThreads,
                                           1u, true, ident);