d75b837ff4
The neutral values for these are -1U, and 0 respectively. We already
have good arithmetic lowerings for selects with one arm equal to these
values. smin/smax are a bit harder, and will be a separate change.
Somewhat surprisingly, this looks to be a net code improvement in all of
the configurations. With both zbb, it's a clear win. With only zicond,
we still seem to come out ahead because we reduce the number of ziconds
needed (since we lower min/max to them). Without either zbb or zicond,
we're a bit more of wash, but the available arithmetic sequences are
good enough that doing the select unconditionally before using branches
for the min/max is probably still worthwhile?
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