Note that i0..i4 need not be consecutive insns, so you'd have to walk the
chain from the location with the death note to the proposed death note site.
If between those locations there's another set of the same pseudo, then drop
the note. Since this may be an expensive check it should probably be
conditionalized on REG_N_SETS (pseudo) > 1
Here is the complicated part. The from_insn is i1, i2/i3 are the
following instructions. The original logic seems to me is scanning
from i3 for one insn for distribution of REG_DEAD note in from_insn.
Since the last use is in from_insn, it makes no sense to scan from i3
(after from_insn). What we need to do is scanning from from_insn in
backward trying to find a place for note distribution. If we run into
a useless set of the note reg, we can just delete that insn or add
REG_UNUSED to it. It just seems not right to do this on instructions
after from_insn, which causes the wrong code in this specific case.