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

[Jay Foad via llvm-commits]

jayfoad 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.

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