[llvm] [TableGen] Fix inconsistent indexing of the RegistersByName table. (PR #161835)

[Owen Anderson via llvm-commits]

resistor wrote:

> > Previously, the initialization of this table in CodeGenRegBank() indexed the table with the name of the tblgen def, while the only user of the table in CompressInstEmitter::validateRegister() indexed it using the name of the tblgen Def.
> 
> Er... what is the difference between "the name of the tblgen def" and "the name of the tblgen Def"?

Sorry, that's an artifact of me revising the commit message. Will fix.

Originally one of them used the `AsmName` and the other used the `Def`'s name.

https://github.com/llvm/llvm-project/pull/161835