[RISCV] Define vsext/vzext intrinsics.

Define vsext/vzext intrinsics.and lower to V instructions.
Define new fraction register class fields in LMULInfo and a
NoReg to present invalid LMUL register classes.

Authored-by: ShihPo Hung <shihpo.hung@sifive.com>
Co-Authored-by: Zakk Chen <zakk.chen@sifive.com>

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D93893