[llvm] [SPARC] Accept named register aliases (PR #212838)

[Kirill A. Korinsky via llvm-commits]

catap wrote:

@koachan it was discovered by attempt to build clisp which failed as:
```
error: <unknown>:0:0: invalid register "%sp" for llvm.read_register
```

https://github.com/llvm/llvm-project/pull/212838