combine: Don't create REG_UNUSED notes if the reg already died (PR99927)

If the register named in an existing REG_UNUSED note dies somewhere
between where the note used to be and I3, we should just drop it.

2021-04-21  Segher Boessenkool  <segher@kernel.crashing.org>

	PR rtl-optimization/99927
	* combine.c (distribute_notes) [REG_UNUSED]: If the register already
	is dead, just drop it.