[RISCV] Split out part of riscv_vector.td to riscv_vector_common.td

This makes other new targets able to reuse predefined classes
in their own *.td files.

Differential Revision: https://reviews.llvm.org/D148680

GitOrigin-RevId: 4bf846ba40f1867889e13596cdcd06ed6f036c7f
2 files changed