[RISCV] Use masked pseudo peephole for reduction pseudos (#71508)

After #71483 we now have a way of marking masked pseudos as having an
unmasked
equivalent, but their mask shouldn't be folded unless it's all ones
since it
would affect the result.

This patch uses it to mark the pseudos for vredsum and friends, which in
turn
allows us to remove the unmasked patterns, and catch some other forms of
vmerge.