[PR80693] drop value of parallel SETs dropped by combine

[PR80693] drop value of parallel SETs dropped by combine

When combine drops a REG_UNUSED SET in a parallel, we have to clear
cached values, so that, even if the REGs remain used (e.g. because
they were referenced in the used SET_SRC), we will not use properties
of the dropped modified value as if they applied to the preserved
original one.

We fail to adjust REG_N_SETS.

for  gcc/ChangeLog

	PR rtl-optimization/80693
	PR rtl-optimization/81019
	PR rtl-optimization/81020
	* combine.c (distribute_notes): Reset any REG_UNUSED REGs that
	are not mentioned in i3.  Place the REG_UNUSED note on i2,
	possibly modified to REG_DEAD, if it did not originate in i3.

for  gcc/testsuite/ChangeLog

	PR rtl-optimization/80693
	PR rtl-optimization/81019
	PR rtl-optimization/81020
	* gcc.dg/pr80693.c: New.
	* gcc.dg/pr81019.c: New.

From-SVN: r255554