rs6000: Refactor altivec_build_resolved_builtin

rs6000: Refactor altivec_build_resolved_builtin

While replacing the built-in machinery, we agreed to defer some necessary
refactoring of the overload processing.  This patch cleans it up considerably.

I've put in one FIXME for an additional level of cleanup that should be done
independently.  The various helper functions (resolve_VEC_*) can be simplified
if we move the argument processing in altivec_resolve_overloaded_builtin
earlier.  But this requires making nontrivial changes to those functions that
will need careful review.  Let's do that in a later patch.

2021-12-16  Bill Schmidt  <wschmidt@linux.ibm.com>

gcc/
	* config/rs6000/rs6000-c.c (resolution): New enum.
	(resolve_vec_mul): New function.
	(resolve_vec_cmpne): Likewise.
	(resolve_vec_adde_sube): Likewise.
	(resolve_vec_addec_subec): Likewise.
	(resolve_vec_splats): Likewise.
	(resolve_vec_extract): Likewise.
	(resolve_vec_insert): Likewise.
	(resolve_vec_step): Likewise.
	(find_instance): Likewise.
	(altivec_resolve_overloaded_builtin): Many cleanups.  Call factored-out
	functions.  Move variable declarations closer to uses.  Add commentary.
	Remove unnecessary levels of braces.  Avoid use of gotos.  Change
	misleading variable names.  Use switches over if-else-if chains.