RISC-V: Add combine optimization by slideup for vec_init vectorization

RISC-V: Add combine optimization by slideup for vec_init vectorization

This patch is a small optimization for vector initialization.
Discovered when I am evaluating benchmarks.

Consider this following case:
void foo3 (int8_t *out, int8_t x, int8_t y)
{
  v16qi v = {y, y, y, y, y, y, y, x, x, x, x, x, x, x, x, x};
  *(v16qi*)out = v;
}

Before this patch:

        vsetivli        zero,16,e8,m1,ta,ma
        vmv.v.x v1,a2
        vslide1down.vx  v1,v1,a1
        vslide1down.vx  v1,v1,a1
        vslide1down.vx  v1,v1,a1
        vslide1down.vx  v1,v1,a1
        vslide1down.vx  v1,v1,a1
        vslide1down.vx  v1,v1,a1
        vslide1down.vx  v1,v1,a1
        vslide1down.vx  v1,v1,a1
        vslide1down.vx  v1,v1,a1
        vse8.v  v1,0(a0)
        ret

After this patch:

	vsetivli	zero,16,e8,m1,ta,ma
	vmv.v.x	v1,a1
	vmv.v.x	v2,a2
	vslideup.vi	v1,v2,8
	vse8.v	v1,0(a0)
	ret

gcc/ChangeLog:

	* config/riscv/riscv-protos.h (enum insn_type): New enum.
	* config/riscv/riscv-v.cc
	(rvv_builder::combine_sequence_use_slideup_profitable_p): New function.
	(expand_vector_init_slideup_combine_sequence): Ditto.
	(expand_vec_init): Add slideup combine optimization.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/vls/def.h: Add combine test.
	* gcc.target/riscv/rvv/autovec/vls-vlmax/combine-1.c: New test.
	* gcc.target/riscv/rvv/autovec/vls/combine-1.c: New test.
	* gcc.target/riscv/rvv/autovec/vls/combine-2.c: New test.
	* gcc.target/riscv/rvv/autovec/vls/combine-3.c: New test.
	* gcc.target/riscv/rvv/autovec/vls/combine-4.c: New test.
	* gcc.target/riscv/rvv/autovec/vls/combine-5.c: New test.
	* gcc.target/riscv/rvv/autovec/vls/combine-6.c: New test.
	* gcc.target/riscv/rvv/autovec/vls/combine-7.c: New test.