rs6000: Handle gimple folding of target built-ins

rs6000: Handle gimple folding of target built-ins

This is another patch that looks bigger than it really is.  Because we
have a new namespace for the builtins, allowing us to have both the old
and new builtin infrastructure supported at once, we need versions of
these functions that use the new builtin namespace.  Otherwise the code is
unchanged.

2021-09-17  Bill Schmidt  <wschmidt@linux.ibm.com>

gcc/
	* config/rs6000/rs6000-call.c (rs6000_gimple_fold_new_builtin): New
	forward decl.
	(rs6000_gimple_fold_builtin): Call rs6000_gimple_fold_new_builtin.
	(rs6000_new_builtin_valid_without_lhs): New function.
	(rs6000_gimple_fold_new_mma_builtin): Likewise.
	(rs6000_gimple_fold_new_builtin): Likewise.