rs6000: Use known constant for GET_MODE_NUNITS and similar

rs6000: Use known constant for GET_MODE_NUNITS and similar

This patch is to clean up some codes with GET_MODE_UNIT_SIZE or
GET_MODE_NUNITS, which can use known constants or just be removed.

Note that Carl Love helped to confirm altivec_vreveti2 introduced
in r12-1341 is useless and can be removed.

gcc/ChangeLog:

	* config/rs6000/altivec.md (altivec_vreveti2): Remove.
	* config/rs6000/vsx.md (*vsx_extract_si, *vsx_extract_si_<uns>float_df,
	*vsx_extract_si_<uns>float_<mode>, *vsx_insert_extract_v4sf_p9): Use
	known constant values to simplify code.