[llvm] [RISCV] Implement RISCV TTI::getNumberOfParts() (PR #223096)

[Ryan Buchner via llvm-commits]

bababuck wrote:

@lukel97 thinking a little more on this, and reading over the use cases again, for the case in the SLP where we are deciding about compressing duplicate values, we don't actually care about the number of registers, we care about if the wider type will cost more to operate on. Which is what `getNumberOfParts` conveys. So I do think that there are cases in the SLP that should be replaced with `getRegUsageForType`, but I think that updating the `RISCV` version is appropriate.

https://github.com/llvm/llvm-project/pull/223096