rs6000: Rework vector integer comparison in rs6000_emit_vector_compare - p3

The current handlings in rs6000_emit_vector_compare is a bit
complicated to me, especially after we emit vector float
comparison insn with the given code directly.  So it's better
to refactor the handlings of vector integer comparison here.

This is part 3, it's to refactor the handlings on NE.
This patch doesn't introduce any functionality change.

gcc/ChangeLog:

	* config/rs6000/rs6000.cc (rs6000_emit_vector_compare): Refactor the
	handlings for operator NE.