RISC-V: Support trailing vec_init optimization

Add optimization when trailing elements > leading elements.

Consider this following case:

#include <stdint.h>

typedef int64_t v16di __attribute__ ((vector_size (128)));

__attribute__ ((noipa)) void
f_v16di (int64_t a, int64_t b, int64_t c, int64_t d, int64_t *out)
{
  v16di v = {a, b, c, d, d, d, d, d, d, d, d, d, d, d, d, d};
  *(v16di *) out = v;
}

https://godbolt.org/z/vWTjbrWGf

Before this patch:

f_v16di:
        vsetivli        zero,16,e64,m8,ta,ma
        vmv.v.x v8,a0
        vslide1down.vx  v8,v8,a1
        vslide1down.vx  v8,v8,a2
        vslide1down.vx  v8,v8,a3
        vslide1down.vx  v8,v8,a3
        vslide1down.vx  v8,v8,a3
        vslide1down.vx  v8,v8,a3
        vslide1down.vx  v8,v8,a3
        vslide1down.vx  v8,v8,a3
        vslide1down.vx  v8,v8,a3
        vslide1down.vx  v8,v8,a3
        vslide1down.vx  v8,v8,a3
        vslide1down.vx  v8,v8,a3
        vslide1down.vx  v8,v8,a3
        vslide1down.vx  v8,v8,a3
        vslide1down.vx  v8,v8,a3
        vse64.v v8,0(a4)
        ret

After this patch:

f_v16di:
	vsetivli	zero,16,e64,m8,ta,ma
	vmv.v.x	v16,a3
	vslide1up.vx	v8,v16,a2
	vslide1up.vx	v16,v8,a1
	vslide1up.vx	v8,v16,a0
	vse64.v	v8,0(a4)
	ret

gcc/ChangeLog:

	* config/riscv/riscv-v.cc (expand_vector_init_trailing_same_elem): New function.
	(expand_vec_init): Add trailing optimization.

gcc/testsuite/ChangeLog:

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