Implement optimization step to replace use of upvalues with constants where upvalue references a constant

[dibyendumajumdar]

Motivation:

local NAME =1

function foo() 
   local t = {}
   t[NAME] = 'Dibyendu'
end

Here NAME is an up-value but we can see it refers to a constant that is never amended. So we can rewrite the code as follows:

function foo() 
   local t = {}
   t[1] = 'Dibyendu'
end

[dibyendumajumdar]

Here is one way to do this:

• Everytime a store is emitted against an up-value mark the local being references as 'updated'
• In a separate pass after IR generation, revisit all up-value references that are constant literals and have not been updated; and replace these with constants

p.s. Need to know that all upvalue access occurs post initialization of local. One option is to insist on statements like:

local NAME = 1

That is, initialized in the local statement, and also all RHS values are constants.

[dibyendumajumdar]

A second more sophisticated approach might be to implement a constant propagation data flow step; but unsure how to handle upvalues there.

[dibyendumajumdar]

IR opcodes that may update a local (directly or via upvalue) are - MOV, GET*, LOADGLOBAL, CLOSURE.
We need to detect if a local was updated post initialization either directly or indirectly via upvalue; a simple way is to do two things: a) mark a variable that is initialized with literal, b) count assignments to the variable