[SYCL] Fix removal of llvm.used for case with spec consts having 2+ u… (#6069)

* [SYCL] Fix removal of llvm.used for case with spec consts having 2+ users

llvm.used array elements may be either bitcasts of spec constants or kernels.
Bitcasts of kernels must be deleted, while bitcasts of spec constants
removal is not required.

This patch removes bitcasts of spec constants only if they have 1 user
and it is safe to remove them.

Signed-off-by: Vyacheslav N Klochkov <vyacheslav.n.klochkov@intel.com>

Author: v-klochkov

llvm/tools/sycl-post-link/sycl-post-link.cpp

@@ -519,12 +519,17 @@ static bool removeSYCLKernelsConstRefArray(GlobalVariable *GV) {
          "Cannot remove initializer of llvm.used global");
   Initializer->destroyConstant();
   for (auto It = IOperands.begin(); It != IOperands.end(); It++) {
-    assert(llvm::isSafeToDestroyConstant(*It) &&
-           "Cannot remove an element of initializer of llvm.used global");
     auto Op = (*It)->getOperand(0);
-    (*It)->destroyConstant();
-    // Remove unused kernel declarations to avoid LLVM IR check fails.
     auto *F = dyn_cast<Function>(Op);
+    if (llvm::isSafeToDestroyConstant(*It)) {
+      (*It)->destroyConstant();
+    } else if (F) {
+      // The element in "llvm.used" array has other users. That is Ok for
+      // specialization constants, but is wrong for kernels.
+      llvm::report_fatal_error("Unexpected usage of SYCL kernel");
+    }
+
+    // Remove unused kernel declarations to avoid LLVM IR check fails.
     if (F && F->isDeclaration())
       F->eraseFromParent();
   }