rs6000: Rework vector float comparison in rs6000_emit_vector_compare - p3

All kinds of vector float comparison operators have been
supported in a rtl comparison pattern as vector.md, we can
just emit an rtx comparison insn with the given comparison
operator in function rs6000_emit_vector_compare instead of
checking and handling the reverse condition cases.

This is part 3, it further checks for comparison opeators
LE/UNGT.  In rs6000_emit_vector_compare, UNGT is handled
with reversed code LE and inverting with one_cmpl_optab,
LE is handled with LT ior EQ, while in vector.md, we have
the support:

; le(a,b)   = ge(b,a)
; ungt(a,b) = ~le(a,b)

The associated test case shows it's an improvement.

gcc/ChangeLog:

	* config/rs6000/rs6000.cc (rs6000_emit_vector_compare): Emit rtx
	comparison for operators LE/UNGT of MODE_VECTOR_FLOAT directly.

gcc/testsuite/ChangeLog:

	* gcc.target/powerpc/vcond-fp.c: New test.