[RISCV] Generalize (sub zext, zext) -> (sext (sub zext, zext)) to add (#86248)

This generalizes the combine added in #82455 to other binary ops,
beginning with adds in this patch.

Because the two zext operands are always +ve when treated as signed, and
we don't get any overflow since the add is carried out in at least N * 2
bits of the narrow type, the result of the add will always be +ve. So we
can use a zext for the outer extend, unlike sub which may produce a -ve
result from two +ve operands.

Although we could still use sext for add, I plan to add support for
other binary ops like mul in a later patch, but mul requires zext to be
correct (because the maximum value will take up the full N * 2 bits). So
I've opted to use zext here too for consistency.

Alive2 proof: https://alive2.llvm.org/ce/z/PRNsUM

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