[turbofan] Fix register constraint for memory barrier.

src/compiler/ppc/instruction-selector-ppc.cc

Index: src/compiler/ppc/instruction-selector-ppc.cc
diff --git a/src/compiler/ppc/instruction-selector-ppc.cc b/src/compiler/ppc/instruction-selector-ppc.cc
index e89506262489db0fa0565240426396571f22d73a..302ae30db411a770203381c6c60850f4df2a0f68 100644
--- a/src/compiler/ppc/instruction-selector-ppc.cc
+++ b/src/compiler/ppc/instruction-selector-ppc.cc
@@ -252,9 +252,7 @@ void InstructionSelector::VisitStore(Node* node) {
       inputs[input_count++] = g.UseUniqueRegister(offset);
       addressing_mode = kMode_MRR;
     }
-    inputs[input_count++] = (write_barrier_kind == kMapWriteBarrier)
-                                ? g.UseRegister(value)
-                                : g.UseUniqueRegister(value);
+    inputs[input_count++] = g.UseUniqueRegister(value);
     RecordWriteMode record_write_mode = RecordWriteMode::kValueIsAny;
     switch (write_barrier_kind) {
       case kNoWriteBarrier: