Fix bad code for x += x.
Fix bad code for x += x.

Beware aliased arguments!  In this case, writing to this->lo_
altered b.lo_ .

I also added a test.  The test fails with old operator+=
and passes with new operator+=.

R=csilvers
DELTA=15  (13 added, 0 deleted, 2 changed)


Revision created by MOE tool push_codebase.
MOE_MIGRATION=2551

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