I found a case where erasing a copy in coalescing causes a subrange to
be modified incorrectly.
It looks like it would be quite a large change to fix this properly,
such that subranges are updated correctly when a subreg is dead in and
live out of a copy being erased.
A possible alternative would be to recalculate subranges from scratch
for a register that is affected by this problem.
For now, I have just disabled that case of erasing a copy when subranges