[RISCV] Move implementation of getVLOpNum and getSEWOpNum from...

[RISCV] Move implementation of getVLOpNum and getSEWOpNum from RISCVInsertVSETVLI to RISCVBaseInfo.h. NFC

We should consolidate the operand counting and ordering into
RISCVBaseInfo.h and stop spreading it around.

Reviewed By: asb

Differential Revision: https://reviews.llvm.org/D125344