#22133)

Fixes mypyc/mypyc#1231

A reference to a local variable compiles to the variable's register, and
the register is only read when the operation that uses the value runs.
If a later operand reassigns the variable with an assignment expression,
the earlier operand sees the new value, so `pair(i, (i := 1))` passed
`(1, 1)` instead of `(0, 1)`. #21249 fixed this for tuple
displays by copying every register item, but calls, method receivers,
operators, list/set/dict displays, subscripts and comprehension
iterables were still affected. The last one could crash: `[x for x in t
if (t := (9,))]` read past the end of the new tuple.

mypyc already collects the locals that assignment expressions reassign
within the current top-level expression (`reassigned_in_expr`, added in
#21702 to restrict borrowing). Now, when one of those locals
is read, its value is copied to a temporary register. Expressions
without such an assignment generate the same IR as before.

A callee reassigned by its own arguments, as in `f(1, (f := g))`, was
also evaluated after the arguments, so `g` was called. Such a callee is
now evaluated first.

Reading a possibly unbound variable before the assignment expression now
raises `UnboundLocalError`, as in Python, instead of using the new
value.

This doesn't change `x += (x := 1)` or `a[(i := 1)] = i`, where the read
happens in statement code outside the expression.