Add zero_extendditi2 for power9.

Add zero_extendditi2 for power9.

This patch provides a zero_extendditi2 pattern for power9 and power10.  On
power8 and previous systems, the generic machine indepenent code will be
used.  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.

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

gcc/
	* config/rs6000/vsx.md (zero_extendditi2): New insn.