eigen

MSVC does not support specializing compound assignments for
`std::complex`, since it already specializes them (contrary to the
standard).

Trying to use one of these on device will currently lead to a
duplicate definition error.  This is still probably preferable
to no error though.  If we remove the definitions for MSVC, then
it will compile, but the kernel will fail silently.

The only proper solution would be to define our own custom `Complex`
type.