LibJS: Make sure that if expressions yield the correct value

When evaluated as an expression "if (true) { 3 } else { 5 }"
should yield 3. This updates the bytecode interpreter to make
it so.

Userland/Libraries/LibJS/Bytecode/Instruction.h

@@ -11,6 +11,7 @@
 
 #define ENUMERATE_BYTECODE_OPS(O) \
     O(Load)                       \
+    O(LoadRegister)               \
     O(Add)                        \
     O(Sub)                        \
     O(Mul)                        \