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

[Jay Foad via llvm-commits]

jayfoad 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"?

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