[turbofan] Fix signature of output operator

This lead to link errors in an unrelated CL. The function was declared
with "const PrintableInstructionBlock&", but defined with
"PrintableInstructionBlock&".

R=mstarzinger@chromium.org

Change-Id: I08a9837dc44dc9d8f508c914da92549179a6790e
Reviewed-on: https://chromium-review.googlesource.com/1088910Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53560}

src/compiler/instruction.cc

@@ -691,7 +691,7 @@ static InstructionBlock* InstructionBlockFor(Zone* zone,
 }
 
 std::ostream& operator<<(std::ostream& os,
-                         PrintableInstructionBlock& printable_block) {
+                         const PrintableInstructionBlock& printable_block) {
   const InstructionBlock* block = printable_block.block_;
   const RegisterConfiguration* config = printable_block.register_configuration_;
   const InstructionSequence* code = printable_block.code_;