[MLIR][OpenMP] Allow the single operation to have a region

[kiranchandramohan]

Summary

The OpenMP standard specifies the region inside to be a structured
block. This means that branches into or out of the region is not
permitted but branches are permitted inside the OpenMP single region.
This is currently modeled by having a region of kind AnyRegion for
all the OpenMP ops. In future we can have a more strictly defined
region.

Fixes #58396.

[peixin]

LGTM. Thanks.

mlir/include/mlir/Dialect/OpenMP/OpenMPOps.td

let description = [{
     in the team, which do not execute the block, wait at an implicit barrier
     at the end of the single construct unless a nowait clause is specified.
   }];
 
   let arguments = (ins Variadic<AnyType>:$allocate_vars,
                        Variadic<AnyType>:$allocators_vars,
                        UnitAttr:$nowait);
 
-  let regions = (region SizedRegion<1>:$region);
+  let regions = (region AnyRegion:$region);
 
   let assemblyFormat = [{
     oilist(`allocate` `(`
               custom<AllocateAndAllocator>(
                 $allocate_vars, type($allocate_vars),
                 $allocators_vars, type($allocators_vars)
               ) `)`
           |`nowait` $nowait

mlir/test/Dialect/OpenMP/ops.mlir

omp.single allocate(%data_var : memref<i32> -> %data_var : memref<i32>) nowait {
       omp.terminator
     }
     // CHECK: omp.terminator
     omp.terminator
   }
   return
 }
 
+// CHECK-LABEL: func @omp_single_multiple_blocks
+func.func @omp_single_multiple_blocks() {
+  // CHECK: omp.single {
+  omp.single {
+    cf.br ^bb2
+    ^bb2:
+    // CHECK: omp.terminator
+    omp.terminator
+  }
+  return
+}
+
 // CHECK-LABEL: @omp_task
 // CHECK-SAME: (%[[bool_var:.*]]: i1, %[[i64_var:.*]]: i64, %[[i32_var:.*]]: i32, %[[data_var:.*]]: memref<i32>)
 func.func @omp_task(%bool_var: i1, %i64_var: i64, %i32_var: i32, %data_var: memref<i32>) {
 
   // Checking simple task
   // CHECK: omp.task {
   omp.task {
     // CHECK: "test.foo"() : () -> ()