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

[Matt Arsenault via llvm-commits]

arsenm 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?

Yes, MCPhysReg is just an int16_t 

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