MIPS64: fix wrong register use in LoadIndexedStringStub.

TEST=mjsunit/string-externalize, others...
BUG=

Review URL: https://codereview.chromium.org/861193008

Cr-Commit-Position: refs/heads/master@{#26278}

src/mips64/code-stubs-mips64.cc

@@ -1347,7 +1347,7 @@ void LoadIndexedStringStub::Generate(MacroAssembler* masm) {
 
   Register receiver = LoadDescriptor::ReceiverRegister();
   Register index = LoadDescriptor::NameRegister();
-  Register scratch = a4;
+  Register scratch = a5;
   Register result = v0;
   DCHECK(!scratch.is(receiver) && !scratch.is(index));
   DCHECK(!FLAG_vector_ics ||