[flang][OpenMP] Skip multi-block teams regions when processing loop directives (llvm#132687)

Fixes a regression when the generic `loop` directive conversion pass
encounters a multi-block `teams` region. At the moment, we skip such
regions.

Author: ergawy

flang/lib/Optimizer/OpenMP/GenericLoopConversion.cpp

@@ -398,8 +398,8 @@ class ReductionsHoistingPattern
 
   static mlir::omp::LoopOp
   tryToFindNestedLoopWithReduction(mlir::omp::TeamsOp teamsOp) {
-    assert(!teamsOp.getRegion().empty() &&
-           teamsOp.getRegion().getBlocks().size() == 1);
+    if (teamsOp.getRegion().getBlocks().size() != 1)
+      return nullptr;
 
     mlir::Block &teamsBlock = *teamsOp.getRegion().begin();
     auto loopOpIter = llvm::find_if(teamsBlock, [](mlir::Operation &op) {

flang/test/Lower/OpenMP/loop-directive.f90

@@ -337,3 +337,24 @@ subroutine loop_teams_loop_reduction
     x = x + i
   end do
 end subroutine
+
+
+! Tests a regression when the pass encounters a multi-block `teams` region.
+subroutine multi_block_teams
+  implicit none
+  integer :: i
+
+  ! CHECK: omp.target {{.*}} {
+  ! CHECK:   omp.teams {
+  ! CHECK:   ^bb1:
+  ! CHECK:     cf.br ^bb2
+  ! CHECK:   ^bb2:
+  ! CHECK:     omp.terminator
+  ! CHECK:   }
+  ! CHECK: }
+  !$omp target teams
+  select case (i)
+  case(1)
+  end select
+  !$omp end target teams
+end subroutine