RISC-V: Implement vector "average" autovec pattern.

RISC-V: Implement vector "average" autovec pattern.

This patch adds vector average patterns

 op[0] = (narrow) ((wide) op[1] + (wide) op[2]) >> 1;
 op[0] = (narrow) ((wide) op[1] + (wide) op[2] + 1) >> 1;

If there is no direct support, the vectorizer can synthesize the pattern
but, presumably, due to lack of narrowing operation support, won't try a
narrowing shift.  Therefore, this patch implements the expanders
instead.

gcc/ChangeLog:

	* config/riscv/autovec.md (<u>avg<v_double_trunc>3_floor):
	Implement expander.
	(<u>avg<v_double_trunc>3_ceil): Ditto.
	* config/riscv/vector-iterators.md (ashiftrt): New iterator.
	(ASHIFTRT): Ditto.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/widen/vec-avg-run.c: New test.
	* gcc.target/riscv/rvv/autovec/widen/vec-avg-rv32gcv.c: New test.
	* gcc.target/riscv/rvv/autovec/widen/vec-avg-rv64gcv.c: New test.
	* gcc.target/riscv/rvv/autovec/widen/vec-avg-template.h: New test.