Improve adding value to array-shape-list

[AndrolGenhald]

Could you just check what is the resulting type if you have a array{0: int, 1: bool, 2: string}<int<3, max>, mixed> for $var and you do a $var[] = 'a'?

Originally posted by @orklah in #8050 (comment)

It becomes non-empty-list<bool|int|mixed|string>, which seems a bit unfortunate since that's strictly less specific than the previous type...