REG_N_REFS vs regmove

[Joern Rennecke]

> If a register is eliminated, but REG_N_REFS is still non-zero, then we end
> up allocating a stack slot for it which will never be used.  This causes
> us to use more stack space than we should, but is otherwise harmless.

It's not just stack space.  There are sometimes a lot more eliminated
registers than remaining ones - and the extra stack space means also
that the ofsets of the remaining stack slots are farther apart - i.e.
it is harder to reload them, and we see less reload inheritance due
to large offsets.