Avoid code duplication in the SEH version of UnwindCursor::getRegisterName. NFC.

This requires making Registers_*::getRegisterName static.

Differential Revision: https://reviews.llvm.org/D55610

llvm-svn: 348981
GitOrigin-RevId: 1ae50897654a474b3171e5dfd2918ef80159cb46
2 files changed