In the degenerate case where we wanted to emit {x = x - x} (where {x}
is any register), ia32 and x64 generated wrong code (producing {-x + -x}
instead). Fix this by special casing this case.

R=ahaas@chromium.org

Bug: chromium:919308
Change-Id: I9cd9818d2a678450ac6530107e7a5cbb625ddb8d
Reviewed-on: https://chromium-review.googlesource.com/c/1405029
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58731}