[risc-v] Apply llvm-prefer-register-over-unsigned from clang-tidy to LLVM

Summary:
This clang-tidy check is looking for unsigned integer variables whose initializer
starts with an implicit cast from llvm::Register and changes the type of the
variable to llvm::Register (dropping the llvm:: where possible).

Depends on D65919

Reviewers: lenary

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Tags: #llvm

Differential Revision for full review was: https://reviews.llvm.org/D65962

llvm-svn: 368629