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

[Lei Huang via llvm-commits]

lei137 wrote:

> @lei137 without my patch:
> 
> ```
> $ llvm-tblgen -gen-register-info -register-info-debug -I include -I lib/Target/PowerPC/ -o /dev/null lib/Target/PowerPC/PPC.td 2>&1 | sed -En '/Register (CARRY|XER):/,/^Register/p'
> Register CARRY:
> 	CostPerUse: 0 
> 	CoveredBySubregs: 0
> 	HasDisjunctSubRegs: 0
> 	RegUnit 1
> 	RegUnit 2
> Register CTR:
> Register XER:
> 	CostPerUse: 0 
> 	CoveredBySubregs: 0
> 	HasDisjunctSubRegs: 0
> 	RegUnit 2
> 	RegUnit 11
> Register ZERO:
> ```
> 
> I.e. CARRY has regunits 1 and 2, XER has regunits 2 and 11, as if they partially overlap.

I am not familiar with the register allocation, so does this mean having this CARRY alias will cause the compiler to generate 1 more regunit then needed?  What kind of side effects does that cause? 

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