Propagate liveness instance properly for ExprAssignOp

In liveness analysis parlance, the lvalue of an assignment operator
expression should be considered as being written, read, *and* used.

Closes #12527.
auto merge of #12733 : edwardw/rust/rw-liveness, r=pcwalton

In liveness analysis parlance, the lvalue of an assignment operator
expression should be considered as being written, read, *and* used.

Closes #12527.