RISC-V: Implement the MASK_LEN_STRIDED_LOAD{STORE}

RISC-V: Implement the MASK_LEN_STRIDED_LOAD{STORE}

This patch would like to implment the MASK_LEN_STRIDED_LOAD{STORE} in
the RISC-V backend by leveraging the vector strided load/store insn.

For example:
void foo (int * __restrict a, int * __restrict b, int stride, int n)
{
    for (int i = 0; i < n; i++)
      a[i*stride] = b[i*stride] + 100;
}

Before this patch:
  38   │     vsetvli a5,a3,e32,m1,ta,ma
  39   │     vluxei64.v  v1,(a1),v4
  40   │     mul a4,a2,a5
  41   │     sub a3,a3,a5
  42   │     vadd.vv v1,v1,v2
  43   │     vsuxei64.v  v1,(a0),v4
  44   │     add a1,a1,a4
  45   │     add a0,a0,a4

After this patch:
  33   │     vsetvli a5,a3,e32,m1,ta,ma
  34   │     vlse32.v    v1,0(a1),a2
  35   │     mul a4,a2,a5
  36   │     sub a3,a3,a5
  37   │     vadd.vv v1,v1,v2
  38   │     vsse32.v    v1,0(a0),a2
  39   │     add a1,a1,a4
  40   │     add a0,a0,a4

The below test suites are passed for this patch:
* The riscv fully regression test.

gcc/ChangeLog:

	* config/riscv/autovec.md (mask_len_strided_load_<mode>): Add
	new pattern for MASK_LEN_STRIDED_LOAD.
	(mask_len_strided_store_<mode>): Ditto but for store.
	* config/riscv/riscv-protos.h (expand_strided_load): Add new
	func decl to expand strided load.
	(expand_strided_store): Ditto but for store.
	* config/riscv/riscv-v.cc (expand_strided_load): Add new
	func impl to expand strided load.
	(expand_strided_store): Ditto but for store.

Signed-off-by: Pan Li <pan2.li@intel.com>
Co-Authored-By: Juzhe-Zhong <juzhe.zhong@rivai.ai>