Optimize method calls, by caching specific function values within the Structure.
The new opcode is used almost like an x86 opcode prefix byte to optimize op_get_by_id,
where the property access is being used to read a function to be passed to op-call (i.e.
'foo.bar();'). This patch modifies the Structure class such that when a property is
put to an object for the first time we will check if the value is a function. If it is,
we will cache the function value on the Structure. A Structure in such a state guarantees
that not only does a property with the given identifier exist on the object, but also that
its value is unchanged. Upon any further attempt to put a property with the same identifier
(but a different value) to the object, it will transition back to a normal Structure (where
it will guarantee the presence but not the value of the property).
op_method_check makes use of the new information made available by the Structure, by
augmenting the functionality of op_get_by_id. Upon generating a FunctionCallDotNode a
check will be emitted prior to the property access reading the function value, and the JIT
will generate an extra (initially unlinked but patchable) set of checks prior to the regular
JIT code for get_by_id. The new code will do inline structure and prototype structure check
(unlike a regular get_by_id, which can only handle 'self' accesses inline), and then performs
an immediate load of the function value, rather than using memory accesses to load the value
from the obejct's property storage array. If the method check fails it will revert, or if
the access is polymorphic, the op_get_by_id will continue to operate - and optimize itself -
just as any other regular op_get_by_id would.
