LibJS/Bytecode: Perform ToNumeric on accumulator before postfix inc/dec

This ensures we get the expected behavior of code like:

    let a = []
    let b = a++

(Where b should be 0, not [], because JavaScript.)

Userland/Libraries/LibJS/Bytecode/Instruction.h

@@ -88,6 +88,7 @@
     O(SuperCall)                     \
     O(Throw)                         \
     O(ThrowIfNotObject)              \
+    O(ToNumeric)                     \
     O(Typeof)                        \
     O(TypeofVariable)                \
     O(UnaryMinus)                    \