PowerPC: rename some functions.

PowerPC: rename some functions.

gcc/
2020-09-11  Michael Meissner  <meissner@linux.ibm.com>

	* config/rs6000/rs6000.c (rs6000_maybe_emit_maxc_minc): Rename
	from rs6000_emit_p9_fp_minmax.  Change return type to bool.  Add
	comments to document NaN/signed zero behavior.
	(rs6000_maybe_emit_fp_cmove): Rename from rs6000_emit_p9_fp_cmove.
	(have_compare_and_set_mask): New helper function.
	(rs6000_emit_cmove): Update calls to new names and the new helper
	function.