Make addti3/subti3 be define_insn_and_split, instead of define_expand

Make addti3/subti3 be define_insn_and_split, instead of define_expand

This patch makes addti3 and subti3 be define_insn_and_split instead of
define_expand.  This patch will be a building block to support in a future
patch PR target/103109 which wants to optimize 128-bit some integer
multiply-add combinations to use the power9 maddld, maddhd, maddhdu
instructions.  In order to support recognizing the multiply and add
combination, we need to keep the addti3 and subti3 as complete insns
through the combiner phase.

2022-03-18   Michael Meissner  <meissner@linux.ibm.com>

gcc/
	* config/rs6000/rs6000.md (addti3): Don't immediate expand the
	insn, delay expansion until the split passes.
	(subti3): Likewise.