MIPS64: 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.

BUG=

Review URL: https://codereview.chromium.org/1066843005

Cr-Commit-Position: refs/heads/master@{#27643}

src/mips64/lithium-codegen-mips64.cc

@@ -175,6 +175,7 @@ bool LCodeGen::GeneratePrologue() {
     Comment(";;; Allocate local context");
     bool need_write_barrier = true;
     // Argument to NewContext is the function, which is in a1.
+    DCHECK(!info()->scope()->is_script_scope());
     if (heap_slots <= FastNewContextStub::kMaximumSlots) {
       FastNewContextStub stub(isolate(), heap_slots);
       __ CallStub(&stub);