[flang][OpenMP] Skip runtime mapping with no offload targets (#144534)

When no offload targets are specified flang will ignore "target"
constructs, but not "target data" constructs. This patch makes the
behavior consistent across all offload-related operations.

While ignoring "target" may produce semantically incorrect code, it may
still be a useful debugging tool.

Author: kparzysz

flang/test/Lower/ignore-target-data.f90

@@ -0,0 +1,30 @@
+!RUN: %flang_fc1 -emit-llvm -fopenmp %s -o - | FileCheck %s --check-prefix=NORT
+!RUN: %flang_fc1 -emit-llvm -fopenmp %s -o - | FileCheck %s --check-prefix=LLVM
+
+!Make sure that there are no calls to the mapper.
+!NORT-NOT: call{{.*}}__tgt_target_data_begin_mapper
+!NORT-NOT: call{{.*}}__tgt_target_data_end_mapper
+
+!Make sure we generate the body
+!LLVM: define internal void @_QFPf(ptr %[[A0:[0-9]+]], ptr %[[A1:[0-9]+]]) {
+!LLVM:   %[[V0:[0-9]+]] = load i32, ptr %[[A0]], align 4
+!LLVM:   %[[V1:[0-9]+]] = load i32, ptr %[[A1]], align 4
+!LLVM:   %[[V2:[0-9]+]] = add i32 %[[V0]], %[[V1]]
+!LLVM:   store i32 %[[V2]], ptr %[[A0]], align 4
+!LLVM:   ret void
+!LLVM: }
+
+
+program test
+
+call f(1, 2)
+
+contains
+
+subroutine f(x, y)
+  integer :: x, y
+  !$omp target data map(tofrom: x, y)
+  x = x + y
+  !$omp end target data
+end subroutine
+end

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

@@ -4378,6 +4378,9 @@ convertOmpTargetData(Operation *op, llvm::IRBuilderBase &builder,
   llvm::OpenMPIRBuilder *ompBuilder = moduleTranslation.getOpenMPBuilder();
   llvm::OpenMPIRBuilder::TargetDataInfo info(/*RequiresDevicePointerInfo=*/true,
                                              /*SeparateBeginEndCalls=*/true);
+  bool isTargetDevice = ompBuilder->Config.isTargetDevice();
+  bool isOffloadEntry =
+      isTargetDevice || !ompBuilder->Config.TargetTriples.empty();
 
   LogicalResult result =
       llvm::TypeSwitch<Operation *, LogicalResult>(op)
@@ -4467,6 +4470,9 @@ convertOmpTargetData(Operation *op, llvm::IRBuilderBase &builder,
 
   if (failed(result))
     return failure();
+  // Pretend we have IF(false) if we're not doing offload.
+  if (!isOffloadEntry)
+    ifCond = builder.getFalse();
 
   using InsertPointTy = llvm::OpenMPIRBuilder::InsertPointTy;
   MapInfoData mapData;