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

[Ryan Buchner via llvm-commits]

bababuck wrote:

> Should SLP be using `getRegUsageForType` in those cases? It's the same interface but it specifically returns the number of registers, not how many times it's split. So it will return the correct result (number of M1 vector registers) for RVV.

Ah I hadn't seen that, thanks!

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