[RISC-V] Simplify codegen for div/mod (#96068)

[RISC-V] Simplify codegen for div/mod (#96068)

* [RISC-V] Simplify codegen for div/mod

After #82924 some testing can be simplified. Change analogous to #85140.

* [RISC-V] Remove sign-extending for divuw/remuw

These instructions take only the lower 32 bits of the argument registers anyway.

* [RISC-V] Give genCodeForDivMod a haircut.

* [RISC-V] Remove TypeGet() from genActualType as per review comments in #95673