[RISCV] Check for COPY_TO_REGCLASS in usesAllOnesMask (#67037)

Sometimes with mask vectors that have been widened, there is a
CopyToRegClass node in between the VMSET and the CopyToReg.

This is a resurrection of https://reviews.llvm.org/D148524, and is
needed to
remove the mask operand when it's extracted from a subvector as planned
in
https://github.com/llvm/llvm-project/pull/66267#discussion_r1331998919