[llvm] [PowerPC] Remove carry register (PR #142420)

[Lei Huang via llvm-commits]

lei137 wrote:

@jayfoad Do you have a use case that shows this?  AFAIK alias means they are exactly the same and it would be interesting to see how aliasing affects the regunits.

> The motivation is to remove a use of Aliases since it would be nice if we could get rid of that in the long term. Also Aliases kind of implies that the two registers carry and xer overlap but are not identical, so several regunits would be generated to represent that. 

https://github.com/llvm/llvm-project/pull/142420