rs6000: Add assert !TARGET_VSX if !TARGET_ALTIVEC and strip a useless check

rs6000: Add assert !TARGET_VSX if !TARGET_ALTIVEC and strip a useless check

In function rs6000_option_override_internal, we have the
checks and adjustments like:

  if (TARGET_P8_VECTOR && !TARGET_ALTIVEC)
    rs6000_isa_flags &= ~OPTION_MASK_P8_VECTOR;

  if (TARGET_P8_VECTOR && !TARGET_VSX)
    rs6000_isa_flags &= ~OPTION_MASK_P8_VECTOR;

But in fact some previous code has guaranteed !TARGET_VSX if
!TARGET_ALTIVEC, so we can remove the former check and
adjustment.  This patch is to remove it accordingly and also
place an explicit assertion.

gcc/ChangeLog:

	* config/rs6000/rs6000.cc (rs6000_option_override_internal): Remove
	useless check on TARGET_P8_VECTOR && !TARGET_ALTIVEC and add an
	assertion on !TARGET_VSX if !TARGET_ALTIVEC.