[RISCV] Simplify tablegen for XCV mac and mul instructions. NFC.

CVInstMac reuses RVInstR that has the same encoding fields.

Add a new class CVInst16I that has specific encoding fields, and two
new class CVInstMac16I and CVInstMul16I that inherite CVInst16I with
different outs and ins.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D156335