rs6000: Rework vector integer comparison in rs6000_emit_vector_compare - p2

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 2, it's to refactor the handlings on LT and LTU.
This patch doesn't introduce any functionality change.

gcc/ChangeLog:

	* config/rs6000/rs6000.cc (rs6000_emit_vector_compare): Refine the
	handlings for operators LT and LTU.