PPC64: [turbofan] Fix DCHECK in AssembleSwap.

Fixes these tests in PPC,debug,non-sim

mjsunit/compiler/osr-maze1
mjsunit/compiler/regress-3
mjsunit/regress/regress-crbug-173907
mjsunit/regress/regress-crbug-173907b

R=mbrandy@us.ibm.com

BUG=

Review URL: https://codereview.chromium.org/1038923003

Cr-Commit-Position: refs/heads/master@{#27488}

src/compiler/ppc/code-generator-ppc.cc

@@ -1380,8 +1380,8 @@ void CodeGenerator::AssembleSwap(InstructionOperand* source,
   } else if (source->IsStackSlot() || source->IsDoubleStackSlot()) {
 #else
   } else if (source->IsStackSlot()) {
-#endif
     DCHECK(destination->IsStackSlot());
+#endif
     Register temp_0 = kScratchReg;
     Register temp_1 = r0;
     MemOperand src = g.ToMemOperand(source);