[Interpreter] Add a simple dead-code elimination bytecode optimizer.

test/cctest/interpreter/bytecode_expectations/ContextVariables.golden

 #
 # Autogenerated by generate-bytecode-expectations.
 #
 
 ---
 pool type: mixed
 execute: yes
 wrap: yes
 
 ---
@@ skipping 87 matching lines @@
 ]
 
 ---
 snippet: "
   'use strict';
   let a = 1;
   { let b = 2; return function() { a + b; }; }
 "
 frame size: 4
 parameter count: 1
-bytecode array length: 48
+bytecode array length: 46
 bytecodes: [
                 B(CallRuntime), U16(Runtime::kNewFunctionContext), R(closure), U8(1),
                 B(PushContext), R(0),
                 B(LdaTheHole),
                 B(StaContextSlot), R(context), U8(4),
   /*   30 E> */ B(StackCheck),
   /*   56 S> */ B(LdaSmi), U8(1),
   /*   56 E> */ B(StaContextSlot), R(context), U8(4),
                 B(LdaConstant), U8(0),
                 B(Star), R(2),
                 B(Mov), R(closure), R(3),
                 B(CallRuntime), U16(Runtime::kPushBlockContext), R(2), U8(2),
                 B(PushContext), R(1),
                 B(LdaTheHole),
                 B(StaContextSlot), R(context), U8(4),
   /*   69 S> */ B(LdaSmi), U8(2),
   /*   69 E> */ B(StaContextSlot), R(context), U8(4),
   /*   72 S> */ B(CreateClosure), U8(1), U8(0),
                 B(PopContext), R(0),
   /*  104 S> */ B(Return),
-                B(PopContext), R(1),
 ]
 constant pool: [
   InstanceType::FIXED_ARRAY_TYPE,
   InstanceType::SHARED_FUNCTION_INFO_TYPE,
 ]
 handlers: [
 ]
 
 ---
 snippet: "
@@ skipping 769 matching lines @@
   /* 3440 E> */ B(Wide), B(StaContextSlot), R16(context), U16(256),
   /* 3445 S> */ B(Wide), B(LdaContextSlot), R16(context), U16(256),
   /* 3454 S> */ B(Return),
 ]
 constant pool: [
   InstanceType::ONE_BYTE_INTERNALIZED_STRING_TYPE,
 ]
 handlers: [
 ]