[Register Coalescer] Fix wrong live-range information with rematerialization.
When rematerializing an instruction that defines a super register that would be
used by a physical subregisters we use the related physical super register for
the definition.
To keep the live-range information accurate, all the defined subregisters must be
marked as dead def, otherwise the register allocation may miss some
interferences.

Working on a reduced test-case!

<rdar://problem/16582185>