[interpreter] Bytecode register optimizer.

test/cctest/interpreter/bytecode_expectations/DoExpression.golden

 #
 # Autogenerated by generate-bytecode-expectations.
 #
 
 ---
 pool type: string
 execute: yes
 wrap: yes
 do expressions: yes
 
 ---
 snippet: "
   var a = do { }; return a;
 "
 frame size: 2
 parameter count: 1
-bytecode array length: 7
+bytecode array length: 8
 bytecodes: [
   /*   30 E> */ B(StackCheck),
-  /*   42 S> */ B(Ldar), R(0),
-                B(Star), R(1),
-  /*   50 S> */ B(Nop),
+  /*   42 S> */ B(Nop),
+                B(Mov), R(0), R(1),
+  /*   50 S> */ B(Ldar), R(1),
   /*   60 S> */ B(Return),
 ]
 constant pool: [
 ]
 handlers: [
 ]
 
 ---
 snippet: "
   var a = do { var x = 100; }; return a;
@@ skipping 26 matching lines @@
 bytecode array length: 25
 bytecodes: [
   /*   30 E> */ B(StackCheck),
   /*   34 E> */ B(StackCheck),
   /*   56 S> */ B(LdaSmi), U8(10),
                 B(Star), R(1),
   /*   69 S> */ B(Inc),
   /*   71 E> */ B(Star), R(1),
                 B(Star), R(0),
   /*   74 S> */ B(Jump), U8(12),
-  /*   64 E> */ B(Ldar), R(0),
-                B(Star), R(1),
+  /*   64 E> */ B(Nop),
+                B(Mov), R(0), R(1),
   /*   84 S> */ B(LdaSmi), U8(20),
   /*   86 E> */ B(Star), R(1),
                 B(Jump), U8(-20),
                 B(LdaUndefined),
   /*   94 S> */ B(Return),
 ]
 constant pool: [
 ]
 handlers: [
 ]