[PowerPC] Don't remove single swap between the load and store

The swap removal pass looks to remove swaps when a loaded value is swapped, some
number of lane-insensitive operations are performed and then the value is
swapped again and stored.

However, in a situation where we load the value, swap it and then store it
without swapping again, the pass erroneously removes the single swap. The
reason is that both checks in the same equivalence class:

- load feeds a swap
- swap feeds a store

pass. However, there is no check that the two swaps are actually a single swap.
This patch just fixes that.

Differential revision: https://reviews.llvm.org/D84785

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