[Bug] OP_SELF uses an unnecessary extra register

["Soni L." <fakedme@...>]

While playing with the Lua parser I noticed OP_SELF uses an unnecessary 
extra register, but this is only visible if you have more than 256 
constants.

This is important for complex functions and expressions, which will have 
one less register available to them.

[soniex2@soniex-pc lua]$ lua selfissue.lua | luac -l -l -o /dev/null - | 
grep -C 4 'SELF'
    768    [257]    LOADK        2 -256    ; "257"
    769    [257]    CALL         1 2 1
    770    [258]    GETUPVAL     1 0    ; _ENV
    771    [258]    LOADK        3 -257    ; "print"
    772    [258]    SELF         1 1 3
    773    [258]    CALL         1 2 1
    774    [258]    RETURN       0 1
constants (257) for 0x132c760:
    1    "2"
[soniex2@soniex-pc lua]$ cat selfissue.lua
local xlim = 257

local x = {"local function _() end\n"}

for i=2,xlim do
  x[i] = "_'"..i.."'\n"
end

table.insert(x, "_ENV:print()\n")

print(table.concat(x))

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