[RISCV] Refine known bits for READ_VLENB

This implements known bits for READ_VALUE using any information known about minimum and maximum VLEN. There's an additional assumption that VLEN is a power of two.

The motivation here is mostly to remove the last use of getMinVLen, but while I was here, I decided to also fix the bug for VLEN < 128 and handle max from command line generically too.

Differential Revision: https://reviews.llvm.org/D128758