[RISCV] Use RISCVISD::SHL_ADD in transformAddShlImm (#89832)

Doing so avoids negative interactions with other combines which don't
know the shl_add is a single instruction. From the commit log, we've had
several combine loops already.

This was originally posted as part of #88791, where a bug was pointed
out. That bug was fixed by #89789 which hits the same issue from another
angle. To confirm the fix, I included the reduced test case here.