[Offload] Guard HSA implicit arguments if they aren't created

Summary:
We conditionally allocate the implicit arguments, so they possibly are
null. The flang compiler seems to hit this case, even though it
shouldn't when it's supposed to conform to the HSA code object. For now
guard this to fix the regression and cover a case in the future where
someone rolls a fully custom implementatation.

Fixes: #132982

Author: jhuber6

offload/plugins-nextgen/amdgpu/src/rtl.cpp

@@ -3386,16 +3386,18 @@ Error AMDGPUKernelTy::launchImpl(GenericDeviceTy &GenericDevice,
     return Err;
 
   // Set the COV5+ implicit arguments to the appropriate values.
-  ImplArgs->BlockCountX = NumBlocks[0];
-  ImplArgs->BlockCountY = NumBlocks[1];
-  ImplArgs->BlockCountZ = NumBlocks[2];
-  ImplArgs->GroupSizeX = NumThreads[0];
-  ImplArgs->GroupSizeY = NumThreads[1];
-  ImplArgs->GroupSizeZ = NumThreads[2];
-  ImplArgs->GridDims = NumBlocks[2] * NumThreads[2] > 1
-                           ? 3
-                           : 1 + (NumBlocks[1] * NumThreads[1] != 1);
-  ImplArgs->DynamicLdsSize = KernelArgs.DynCGroupMem;
+  if (ImplArgs) {
+    ImplArgs->BlockCountX = NumBlocks[0];
+    ImplArgs->BlockCountY = NumBlocks[1];
+    ImplArgs->BlockCountZ = NumBlocks[2];
+    ImplArgs->GroupSizeX = NumThreads[0];
+    ImplArgs->GroupSizeY = NumThreads[1];
+    ImplArgs->GroupSizeZ = NumThreads[2];
+    ImplArgs->GridDims = NumBlocks[2] * NumThreads[2] > 1
+                             ? 3
+                             : 1 + (NumBlocks[1] * NumThreads[1] != 1);
+    ImplArgs->DynamicLdsSize = KernelArgs.DynCGroupMem;
+  }
 
   // Push the kernel launch into the stream.
   return Stream->pushKernelLaunch(*this, AllArgs, NumThreads, NumBlocks,