[Flang][OpenMP][MLIR] Remove deletion of unused declare target global after use replacement (#67762)

At the moment, for device a reference pointer is generated in place of
the original declare target global value, this reference pointer is the
pointer that actually receives the data. In Clang the original global
value isn't generated for device, just the reference pointer.

Unfortunately for Flang/MLIR this is currently not the case, as the
declare target attribute is processed after the creation of the global
so we end up with a dead global on device effectively after rewriting
its uses to the new device reference pointer.

It appears I was a little overzealous with the deletion of the declare
target globals for device. The current method breaks in-cases where the
same declare target global is used across two target regions (added a
runtime reproduced in the patch). As it'll effectively delete it before
the second target gets a chance to be written to LLVM IR and have it's
uses rewritten .

I'd like to remove this deletion as the dead global isn't breaking any
code and will likely be removed in later dead code elimination passes,
perhaps a little too heavy handed with the original approach.

Author: agozillon

mlir/lib/Target/LLVMIR/Dialect/OpenMP/OpenMPToLLVMIRTranslation.cpp

@@ -1989,15 +1989,6 @@ handleDeclareTargetMapVar(llvm::ArrayRef<Value> mapOperands,
           user->replaceUsesOfWith(mapOpValue, load);
         }
       }
-
-      // A global has already been generated by this stage for device
-      // that's now dead after the remapping, we can remove it now,
-      // as we have replaced all usages with the new ref pointer.
-      if (llvm::GlobalValue *unusedGlobalVal =
-              dyn_cast<llvm::GlobalValue>(mapOpValue)) {
-        unusedGlobalVal->dropAllReferences();
-        unusedGlobalVal->eraseFromParent();
-      }
     }
   }
 }

openmp/libomptarget/test/offloading/fortran/double-target-call-with-declare-target.f90

@@ -0,0 +1,43 @@
+! Offloading test with two target regions mapping the same
+! declare target Fortran array and writing some values to 
+! it before checking the host correctly receives the 
+! correct updates made on the device.
+! REQUIRES: flang, amdgcn-amd-amdhsa
+! UNSUPPORTED: nvptx64-nvidia-cuda
+! UNSUPPORTED: nvptx64-nvidia-cuda-LTO
+! UNSUPPORTED: aarch64-unknown-linux-gnu
+! UNSUPPORTED: aarch64-unknown-linux-gnu-LTO
+! UNSUPPORTED: x86_64-pc-linux-gnu
+! UNSUPPORTED: x86_64-pc-linux-gnu-LTO
+
+! RUN: %libomptarget-compile-fortran-run-and-check-generic
+module test_0
+    implicit none
+    integer :: sp(10) = (/0,0,0,0,0,0,0,0,0,0/)
+    !$omp declare target link(sp)
+end module test_0
+
+program main
+    use test_0
+    integer :: i = 1
+    integer :: j = 11
+
+!$omp target map(tofrom:sp) map(to: i, j)
+    do while (i <= j)
+        sp(i) = i;
+        i = i + 1
+    end do
+!$omp end target
+
+!$omp target map(tofrom:sp) map(to: i, j)
+    do while (i <= j)
+        sp(i) = sp(i) + i;
+        i = i + 1
+    end do
+!$omp end target
+    
+print *, sp(:)
+
+end program
+
+! CHECK: 2 4 6 8 10 12 14 16 18 20