Fix edge case with many arguments

[SquidDev]

There was an issue with code similar to this:

local function arg(a, b, ...)
    if a then error() end
    b = b + 1 -- Error occurs here
end

The issue lies in the code being a VarArgFunction, and so variables have to be extracted into slots. However the slot variable for b is never set as it doesn't think the argument is being used.

You can see from the block breakdown that block 5-6 has 2 previous blocks. As a result it didn't know which variable to use and so it never said that the argument was used.

proto 'EdgeCases_LuaCompiled$0'
    block 1-2  nxt: (3,5)
         0.p    1.p    2.p    3.p        1  [-]  TEST  0 0 0
         0.p    1.p    2.p    3.p        2  [4]  JMP  2  ; to 5
    block 3-4  prv: (2)  nxt: (5)
         0.p    1.p    2.p    3.2        3  [4]  GETGLOBAL  3 -1  ; "error"
         0.p    1.p    2.p    x.x        4  [4]  CALL  3 1 1
    block 5-6  prv: (4,2)
         0.p    1.4    2.p    x.x        5  [5]  ADD  1 1 -2  ; - 1
         0.p    1.4    2.p    x.x        6  [6]  RETURN  0 1

The fix checks that if all slots in previous blocks are the same then this slot must be equal to that one.

[SquidDev]

This isn't ready to be merged as the fix will not address issues such as:

local function func(a, b, ...)
    if a then b = 0 end
    b = b + 1
end

Having read the code a bit more, most of this is already implemented within PhiVarInfo, however the PhiVarInfo is disposed as we assign it to the same variable.

[SquidDev]

The fix forcibly resolves those PhiVarInfo instances not already resolved. PhiVarInfo instances will always bubble up their isreferenced settings.