[llvm] [PowerPC] Optimize allocation of Conditional Register (PR #69299)

[Kai Luo via llvm-commits]

bzEq wrote:

>  tell the register allocator to use a circular allocation order for a particular register class

I've got through the code of GreedyRA, this might not help a lot, since the order of allocating virtual registers is determined by weight, not the definition order inside one MBB. That's to say there is still possibility adjacent CR bit definition might be allocating the same CR even in circular allocation order.

https://github.com/llvm/llvm-project/pull/69299