[AArch64] Fix a minor issue with AArch64LoopIdiomTransform (#78136)

I found another case where in the end block we could have a PHI that we
deal with incorrectly. The two incoming values are unique - one of them
is
the induction variable and another one is a value defined outside the
loop, e.g.

  %final_val = phi i32 [ %inc, %while.body ], [ %d, %while.cond ]

We won't correctly select between the two values in the new end block
that
we create and so we will get the wrong result.