[RISCV] Use RISCV::RVVBitsPerBlock for RGK_ScalableVector in getRegisterBitWidth.

I might be wrong, but I think this is should be width of the known
min size we use for scalable vectors. It shouldn't scale with
minimum vlen.

Reviewed By: frasercrmck

Differential Revision: https://reviews.llvm.org/D107945