Fix evaluation order in emulation of bitfield assignment

A bitfield assignment `x.b = f()` is expanded into a read-modify-write
on `x.carrier`.  Wrong results can occur if `x.carrier` is read before
the call to `f()`, and `f` itself modifies a bitfield with the same
carrier `x.carrier`.

In this temporary fix, we play on the evaluation order implemented by
the SimplExpr pass of CompCert (left-to-right for side-effecting
subexpression) to make sure the read part of the read-modify-write
sequence occurs after the evaluation of the right-hand side.

More substantial fixes will be considered later.

Fixes: #395

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