[RISCV] Rewrite RISCVCodeGenPrepare using zext nneg [nfc-ish] (#70739)

This stacks on #70725. Once we have lowering for zext nneg, we can
rewrite all of the existing RISCVCodeGenPrepare login in terms of zext
nneg instead of sext. The change isn't NFC from the perspective of the
individual pass, but should be from the perspective of codegen as a
whole.

As noted in the TODO, one piece can be moved to instcombine, but I'll
leave that to a separate commit.