rs6000: Rework vector integer comparison in rs6000_emit_vector_compare - p5

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 5, it's to refactor all the handlings of vector
integer comparison to make it neat.  This patch doesn't
introduce any functionality change.

gcc/ChangeLog:

	* config/rs6000/rs6000.cc (rs6000_emit_vector_compare): Refactor the
	handlings of vector integer comparison.