Implement escape analysis to improve performance

[dibyendumajumdar]

By doing escape analysis it should be possible to determine if a local variable can escape from a function. This would allow variables that are of primitive type to use the JIT function stack rather than Lua stack.

As per Roberto - only possible way for a variable to escape is via closures being created. So we need to scan the up-values of a closure to determine if any locals are escaping. All up-values get flattened (i.e. any up-values up the chain gets referenced in the closure being created) - so searching the closure's list of up-values is enough.

Example:

> function x()
>>   local i: integer
>>   function y()
>>     local j: integer
>>     function z()
>>       return i+j
>>     end
>>   end
>> end
> ravi.dumplua(x)

function <stdin:1,9> (5 instructions at 0x7ffc4500fac0)
0 params, 2 slots, 1 upvalue, 1 local, 1 constant, 1 function
    1   [2] LOADNIL     0 0
    2   [2] LOADIZ      0
    3   [8] CLOSURE     1 0 ; 0x7ffc4500fdb0
    4   [3] SETTABUP    0 -1 1  ; _ENV "y"
    5   [9] RETURN      0 1
constants (1) for 0x7ffc4500fac0:
    1   "y"
locals (1) for 0x7ffc4500fac0:
    0   i   3   6
upvalues (1) for 0x7ffc4500fac0:
    0   _ENV    0   0

function <stdin:3,8> (5 instructions at 0x7ffc4500fdb0)
0 params, 2 slots, 2 upvalues, 1 local, 1 constant, 1 function
    1   [4] LOADNIL     0 0
    2   [4] LOADIZ      0
    3   [7] CLOSURE     1 0 ; 0x7ffc4500ff60
    4   [5] SETTABUP    0 -1 1  ; _ENV "z"
    5   [8] RETURN      0 1
constants (1) for 0x7ffc4500fdb0:
    1   "z"
locals (1) for 0x7ffc4500fdb0:
    0   j   3   6
upvalues (2) for 0x7ffc4500fdb0:
    0   _ENV    0   0
    1   i   1   0

function <stdin:5,7> (5 instructions at 0x7ffc4500ff60)
0 params, 2 slots, 2 upvalues, 0 locals, 0 constants, 0 functions
    1   [6] GETUPVAL    0 0 ; i
    2   [6] GETUPVAL    1 1 ; j
    3   [6] ADDII       0 0 1
    4   [6] RETURN      0 2
    5   [7] RETURN      0 1
constants (0) for 0x7ffc4500ff60:
locals (0) for 0x7ffc4500ff60:
upvalues (2) for 0x7ffc4500ff60:
    0   i   0   1
    1   j   1   0
>