fix deprecated assign update expression for nested expressions

[Dhairya Patel (HABER7789)]

Fix deprecated assign-update quick fix for nested expressions

The DeprecatedAssignUpdateExpr quick fix flattens nested w/= updates by matching index source text. That's unsafe for impure indices: the flattened form evaluates the index fewer times than the original, changing behavior.

Now it only merges levels where the shared indices are pure, otherwise falling back to the safe single-level rewrite. The assigned value is never required to be pure (it's evaluated once either way).

Example (NextIndex() returns a different value each call):

a w/= NextIndex() <- (a[NextIndex()] w/ 1 <- 7)

before →  a[NextIndex()][1] = 7                     // runs NextIndex() once, wrong
after  →  a[NextIndex()] = a[NextIndex()] w/ 1 <- 7 // both calls preserved

Pure cases still flatten fully, e.g. a w/= 0 <- (a[0] w/ 1 <- 7) → a[0][1] = 7.

[Dhairya Patel (HABER7789)]

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