[llvm] [RISCV] Cost UDIV/UREM by a constant power of 2 as a SHL/AND in getArithmeticInstrCost() (PR #179570)

[Ryan Buchner via llvm-commits]

bababuck wrote:

> We can just put it in RISC-V.

Thanks! Rebased and updated to RISCV only. I would still like to move this to a generic function in the future, as would help with cases such as #188572.  Do you have any advice for getting a TTI change like that approved across all backends? 

https://github.com/llvm/llvm-project/pull/179570