[llvm] [llvm][PowerPC] Correct handling of spill slots for SPE when EXPENSIVE_CHECKS is enabled (PR #73940)

[David Spickett via llvm-commits]

DavidSpickett wrote:

@arsenm On https://reviews.llvm.org/D152437 you said:
"Don't use reference to MCPhysReg"

I can address that too if you want. Looks like MCPhysReg is passed around by copy is that right?

https://github.com/llvm/llvm-project/pull/73940