PPC: [turbofan] Canonicalize return sequence for JSFunctions.

Port 2b9112a5

Original commit message:
This optimization is already implemented in fullcodegen, and
basically makes sure that we do not unecessarily blow up the
code with duplicated return sequences everywhere.

R=bmeurer@chromium.org, dstence@us.ibm.com, michael_dawson@ca.ibm.com
BUG=

Review URL: https://codereview.chromium.org/1218493003

Cr-Commit-Position: refs/heads/master@{#29333}

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

@@ -1372,11 +1372,17 @@ void CodeGenerator::AssembleReturn() {
     __ LeaveFrame(StackFrame::MANUAL);
     __ Ret();
   } else if (descriptor->IsJSFunctionCall() || needs_frame_) {
+    // Canonicalize JSFunction return sites for now.
+    if (return_label_.is_bound()) {
+      __ b(&return_label_);
+    } else {
+      __ bind(&return_label_);
       int pop_count = descriptor->IsJSFunctionCall()
                           ? static_cast<int>(descriptor->JSParameterCount())
                           : 0;
       __ LeaveFrame(StackFrame::MANUAL, pop_count * kPointerSize);
       __ Ret();
+    }
   } else {
     __ Ret();
   }