[RISCV] Implement isElementTypeLegalForScalableVector TTI hook

This brings us into alignment with AArch64, and in the process fixes a compiler crash bug in uniform store handling in the vectorizer.

Before the recent invalid cost bailout work, this would have also avoided crashes on invalid costs in some cases. I honestly think the vectorizer should gracefully bailout on uniform stores it can't use a scatter for, but it doesn't, so lets take the path of least resistance here. It's also possible that there are other vectorizer bugs AArch64 isn't seeing because of this hook; we don't want to be finding them either.

Differential Revision: https://reviews.llvm.org/D127514