[RISCV] Convert registers from unsigned to Register

[RISCV] Convert registers from unsigned to Register

Only in public interfaces that have not yet been converted should there remain
registers with unsigned type.

Differential Revision: https://reviews.llvm.org/D66252

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@369114 91177308-0d34-0410-b5e6-96231b3b80d8