[RISCV] Add codegen support for RISCV XTHeadVdot Extension

This patch adds codegen support for XTHeadVdot extension,
which is documented here, it's based on standard vector extension v1.0:
  https://github.com/T-head-Semi/thead-extension-spec

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D140771