[PPCMIPeephole] Fix incorrect compare elimination

D38236 moves a redundant compare instruction from the loop body to the
preheader.

It has a bug: when `MBB1 == &MBB2`, there may be only one compare instruction in the
loop. The code will lift the compare instruction to the preheader, failing to
account for the change of the compare result in a tail call, leading to a miscompile.

Suppress the compare elimination to fix https://github.com/llvm/llvm-project/issues/62294

Reviewed By: #powerpc, nemanjai

Differential Revision: https://reviews.llvm.org/D149030