[llvm] [PowerPC] don't eliminate the signext if the input is zero extended (PR #84419)

[Kai Luo via llvm-commits]

bzEq wrote:

I'm not really catching where miscompilation occurs, could you please elaborate? To my understanding, the original transformation looks correct, https://alive2.llvm.org/ce/z/Vtb2Ux.

https://github.com/llvm/llvm-project/pull/84419