Assignment expressions in `if`-statements don't correctly promote nullable variables about language

[eernstg]

There is a request for generalisation of the flow analysis in #1224, in order to get a larger set of reasons for promoting a local variable. In that issue the topic is that if (foo?.something == somethingNotNull) ... could promote foo to have a non-nullable type in the true continuation (because the condition would necessarily be false when foo == null).

This issue is similar in that it is a request for detecting that x can't be null in the true continuation of (x = e) != null.

I'll move this issue to the language repository and label it in a way which is similar to the existing issue.

[eernstg]

Maybe we'd just create a new label (e.g., enhanced-promotion) and then use that for proposals about new ways to establish that a given promotion is justified.

[eernstg]

@stereotype441, @johnniwinther, it seems that we may have a group of somewhat similar requests in this area (at least, #1224 seems related). Would you prefer to make it a single issue covering all the variants? What's the best labeling?

[lrhn]

A single meta-issue with links to individual issues for concrete enhancement ideas is probably better than combining different ideas into one issue, in case we choose to do some, but not all, of them.