[PowerPC] Fix infinite loop in peephole CR optimization (PR49509)

If we encounter a degenerate select node where both operands are
the same, then we can continue negating the condition while swapping
operands, resulting in an infinite loop. Avoid this by bailing out
if both operands are the same.

Fixes https://bugs.llvm.org/show_bug.cgi?id=49509.

Differential Revision: https://reviews.llvm.org/D98340