[RISCV] Move RISCV::RVVBitsPerBlock from TargetParser to Support/RISCVISAInfo.h.

RISCVTargetParser.h has a dependency on a tablegen generated file.

Using RISCVISAInfo.h instead avoids this dependency.

We just need this constant somewhere visible to the frontend and
backend and I'm trying to avoid adding a header just for it.