PPC: Fix DCHECK in CallApiFunctionStubHelper.

R=joransiu@ca.ibm.com, jyan@ca.ibm.com, michael_dawson@ca.ibm.com
BUG=

Review URL: https://codereview.chromium.org/1756303002

Cr-Commit-Position: refs/heads/master@{#34448}

src/ppc/code-stubs-ppc.cc

@@ -5684,7 +5684,7 @@ static void CallApiFunctionStubHelper(MacroAssembler* masm,
   STATIC_ASSERT(FCA::kHolderIndex == 0);
   STATIC_ASSERT(FCA::kArgsLength == 7);
 
-  DCHECK(argc.is_immediate() || r3.is(argc.reg()));
+  DCHECK(argc.is_immediate() || r6.is(argc.reg()));
 
   // context save
   __ push(context);