PPC: Make --always-opt also optimize top-level code.

Port 2d281e71

Original commit message:
This enables eager optimization of top-level code with TurboFan and
extends test coverage by triggering it with the --always-opt flag.
Script contexts are now also properly allocated in TurboFan.

R=titzer@chromium.org, mstarzinger@chromium.org,mbrandy@us.ibm.com
BUG=

Review URL: https://codereview.chromium.org/1052123005

Cr-Commit-Position: refs/heads/master@{#27726}

src/ppc/lithium-codegen-ppc.cc

@@ -182,6 +182,7 @@ bool LCodeGen::GeneratePrologue() {
     Comment(";;; Allocate local context");
     bool need_write_barrier = true;
     // Argument to NewContext is the function, which is in r4.
+    DCHECK(!info()->scope()->is_script_scope());
     if (heap_slots <= FastNewContextStub::kMaximumSlots) {
       FastNewContextStub stub(isolate(), heap_slots);
       __ CallStub(&stub);