Peephole optimization for double load to registers

[jackdotink]

For the following code in O2 mode, Luau creates this bytecode:

local v
v = 10

LOADNIL R0
LOADN R0 10

Here Luau is assigning to the same register twice in a row. It's possible that Luau can remove this in an optimization pass to keep only the latter load. My own personal motivation for this is zap, which uses the above syntax extensively, and it wouldn't be trivial to change that.