[PowerPC] Fix emitting instruction pairs on LE

My patch r204634 to emit instructions in little-endian format failed to
handle those special cases where we emit a pair of instructions from a
single LLVM MC instructions (like the bl; nop pairs used to implement
the call sequence).

In those cases, we still need to emit the "first" instruction (the one
in the more significant word) first, on both big and little endian,
and not swap them.