Improve checking LHS of Assign

[som-snytt]

Fixes #22970

Simple example showing how awkward this is compared to a traversal, where there is more control when descending.

Previously, it "ignored" the LHS of Assign to avoid taking x = y as ref to x instead of assignment to.

Now, "mark" the LHS as assignment, so transformIdent and Select set a "mode" flag, such that x is correctly taken as either a ref or assign to x.

Probably there is more complex syntax to handle as LHS, but I haven't had coffee yet.