rs6000: Fix regression cases caused 16-byte by pieces move

rs6000: Fix regression cases caused 16-byte by pieces move

The previous patch enables 16-byte by pieces move. Originally 16-byte
move is implemented via pattern.  expand_block_move does an optimization
on P8 LE to leverage V2DI reversed load/store for memory to memory move.
Now 16-byte move is implemented via by pieces move and finally split to
two DI load/store.  This patch creates an insn_and_split pattern to
retake the optimization.

gcc/
	PR target/111449
	* config/rs6000/vsx.md (*vsx_le_mem_to_mem_mov_ti): New.

gcc/testsuite/
	PR target/111449
	* gcc.target/powerpc/pr111449-2.c: New.