RISC-V: Refine unsigned avg_floor/avg_ceil

This patch is inspired by LLVM patches:
https://github.com/llvm/llvm-project/pull/76550
https://github.com/llvm/llvm-project/pull/77473

Use vaaddu for AVG vectorization.

Before this patch:

        vsetivli        zero,8,e8,mf2,ta,ma
        vle8.v  v3,0(a1)
        vle8.v  v2,0(a2)
        vwaddu.vv        v1,v3,v2
        vsetvli zero,zero,e16,m1,ta,ma
        vadd.vi v1,v1,1
        vsetvli zero,zero,e8,mf2,ta,ma
        vnsrl.wi        v1,v1,1
        vse8.v  v1,0(a0)
        ret

After this patch:

	vsetivli	zero,8,e8,mf2,ta,ma
	csrwi	vxrm,0
	vle8.v	v1,0(a1)
	vle8.v	v2,0(a2)
	vaaddu.vv	v1,v1,v2
	vse8.v	v1,0(a0)
	ret

Note on signed averaging addition

Based on the rvv spec, there is also a variant for signed averaging addition called vaadd.
But AFAIU, no matter in which rounding mode, we cannot achieve the semantic of signed averaging addition through vaadd.
Thus this patch only introduces vaaddu.

More details in:
https://github.com/riscv/riscv-v-spec/issues/935
https://github.com/riscv/riscv-v-spec/issues/934

Tested on both RV32 and RV64 no regression.

Ok for trunk ?

gcc/ChangeLog:

	* config/riscv/autovec.md (<u>avg<v_double_trunc>3_floor): Remove.
	(avg<v_double_trunc>3_floor): New pattern.
	(<u>avg<v_double_trunc>3_ceil): Remove.
	(avg<v_double_trunc>3_ceil): New pattern.
	(uavg<mode>3_floor): Ditto.
	(uavg<mode>3_ceil): Ditto.
	* config/riscv/riscv-protos.h (enum insn_flags): Add for average addition.
	(enum insn_type): Ditto.
	* config/riscv/riscv-v.cc: Ditto.
	* config/riscv/vector-iterators.md (ashiftrt): Remove.
	(ASHIFTRT): Ditto.
	* config/riscv/vector.md: Add VLS modes.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/vls/avg-1.c: Adapt test.
	* gcc.target/riscv/rvv/autovec/vls/avg-2.c: Ditto.
	* gcc.target/riscv/rvv/autovec/vls/avg-3.c: Ditto.
	* gcc.target/riscv/rvv/autovec/vls/avg-4.c: Ditto.
	* gcc.target/riscv/rvv/autovec/vls/avg-5.c: Ditto.
	* gcc.target/riscv/rvv/autovec/vls/avg-6.c: Ditto.
	* gcc.target/riscv/rvv/autovec/widen/vec-avg-rv32gcv.c: Ditto.
	* gcc.target/riscv/rvv/autovec/widen/vec-avg-rv64gcv.c: Ditto.