[RISCV] Add ISel patterns for scalable mask exts & truncs

Original patch by @rogfer01.

This patch adds support for sign-, zero-, and any-extension from
scalable mask vector types to integer vector types, as well as
truncation in the opposite direction.

Authored-by: Roger Ferrer Ibanez <rofirrim@gmail.com>
Co-Authored-by: Fraser Cormack <fraser@codeplay.com>

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D94590