Always memoize calls in lhs of coalesce assignment

We don't want to invoke calls twice, even if they are considered "variables",
i.e. might be writable if returning a reference. Function

Always memoize calls in lhs of coalesce assignment

We don't want to invoke calls twice, even if they are considered "variables",
i.e. might be writable if returning a reference. Function calls behave the same
in all BP contexts so they don't need to be invoked twice. The singular
exception to this is nullsafe coalesce in isset/empty, because it needs to
return false/true respectively when short-circuited. However, since nullsafe
calls are not allwed in write context we may ignore this problem.

Closes GH-11592

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