[OpenMP] Simplify code for reductions on distribute directives, NFC.

[Hahnfeld]

Summary

Only need to care about the 'distribute simd' case, all other composite
directives are handled elsewhere. This was already reflected in the
outer 'if' condition, so all other inner conditions could never be true.

[ABataev]

LG

lib/CodeGen/CGStmtOpenMP.cpp

// Emit 'then' code.
         EmitOMPSimdFinal(S, [IL, &S](CodeGenFunction &CGF) {
           return CGF.Builder.CreateIsNotNull(
               CGF.EmitLoadOfScalar(IL, S.getBeginLoc()));
         });
       }
       if (isOpenMPSimdDirective(S.getDirectiveKind()) &&
           !isOpenMPParallelDirective(S.getDirectiveKind()) &&
           !isOpenMPTeamsDirective(S.getDirectiveKind())) {
-        OpenMPDirectiveKind ReductionKind = OMPD_unknown;
-        if (isOpenMPParallelDirective(S.getDirectiveKind()) &&
-            isOpenMPSimdDirective(S.getDirectiveKind())) {
-          ReductionKind = OMPD_parallel_for_simd;
-        } else if (isOpenMPParallelDirective(S.getDirectiveKind())) {
-          ReductionKind = OMPD_parallel_for;
-        } else if (isOpenMPSimdDirective(S.getDirectiveKind())) {
-          ReductionKind = OMPD_simd;
-        } else if (!isOpenMPTeamsDirective(S.getDirectiveKind()) &&
-                   S.hasClausesOfKind<OMPReductionClause>()) {
-          llvm_unreachable(
-              "No reduction clauses is allowed in distribute directive.");
-        }
-        EmitOMPReductionClauseFinal(S, ReductionKind);
+        EmitOMPReductionClauseFinal(S, OMPD_simd);
         // Emit post-update of the reduction variables if IsLastIter != 0.
         emitPostUpdateForReductionClause(
             *this, S, [IL, &S](CodeGenFunction &CGF) {
               return CGF.Builder.CreateIsNotNull(
                   CGF.EmitLoadOfScalar(IL, S.getBeginLoc()));
             });
       }
       // Emit final copy of the lastprivate variables if IsLastIter != 0.