[interpreter] Introduce fused bytecodes for common sequences.

test/cctest/interpreter/bytecode_expectations/DoExpression.golden

 #
 # Autogenerated by generate-bytecode-expectations.
 #
 
 ---
 pool type: string
 execute: yes
 wrap: yes
 do expressions: yes
 
@@ skipping 15 matching lines @@
 ]
 handlers: [
 ]
 
 ---
 snippet: "
   var a = do { var x = 100; }; return a;
 "
 frame size: 3
 parameter count: 1
-bytecode array length: 13
+bytecode array length: 14
 bytecodes: [
   /*   30 E> */ B(StackCheck),
   /*   55 S> */ B(LdaSmi), U8(100),
                 B(Star), R(1),
-  /*   42 S> */ B(LdaUndefined),
-                B(Star), R(0),
+  /*   42 S> */ B(LdrUndefined), R(0),
+                B(Ldar), R(0),
   /*   42 E> */ B(Nop),
                 B(Star), R(2),
   /*   63 S> */ B(Nop),
   /*   73 S> */ B(Return),
 ]
 constant pool: [
 ]
 handlers: [
 ]
 
@@ skipping 19 matching lines @@
   /*   86 E> */ B(Star), R(1),
                 B(Jump), U8(-20),
                 B(LdaUndefined),
   /*   94 S> */ B(Return),
 ]
 constant pool: [
 ]
 handlers: [
 ]