[perf] avoid double memory usage in append opcodes

[lifeart]

No description provided.

[NullVoxPopuli]

This one def wants to wait on #1501
(I think)

[NullVoxPopuli]

@lifeart did you have numbers on this one? I think something this focused is fine to land before error recovery, as the rebase situation wouldn't be too bad over there

[lifeart]

@NullVoxPopuli no numbers here, I think we could count it as cleanup from IE11 support, because Array.fill not working in 11 - https://caniuse.com/mdn-javascript_builtins_array_fill
And list of functions is created to simulate this.

Most of usages is static things (initialized once), we unable to catch 'difference'.

Only improvement kinda here

(looks like it's a mistake in code and .slice(); is some left copy-pasta from somewhere.