RISC-V: Support form 1 of integer scalar .SAT_ADD

This patch would like to support the scalar signed ssadd pattern
for the RISC-V backend.  Aka

Form 1:
  #define DEF_SAT_S_ADD_FMT_1(T, UT, MIN, MAX) \
  T __attribute__((noinline))                  \
  sat_s_add_##T##_fmt_1 (T x, T y)             \
  {                                            \
    T sum = (UT)x + (UT)y;                     \
    return (x ^ y) < 0                         \
      ? sum                                    \
      : (sum ^ x) >= 0                         \
        ? sum                                  \
        : x < 0 ? MIN : MAX;                   \
  }

DEF_SAT_S_ADD_FMT_1(int64_t, uint64_t, INT64_MIN, INT64_MAX)

Before this patch:
  10   │ sat_s_add_int64_t_fmt_1:
  11   │     mv   a5,a0
  12   │     add  a0,a0,a1
  13   │     xor  a1,a5,a1
  14   │     not  a1,a1
  15   │     xor  a4,a5,a0
  16   │     and  a1,a1,a4
  17   │     blt  a1,zero,.L5
  18   │     ret
  19   │ .L5:
  20   │     srai a5,a5,63
  21   │     li   a0,-1
  22   │     srli a0,a0,1
  23   │     xor  a0,a5,a0
  24   │     ret

After this patch:
  10   │ sat_s_add_int64_t_fmt_1:
  11   │     add  a2,a0,a1
  12   │     xor  a1,a0,a1
  13   │     xor  a5,a0,a2
  14   │     srli a5,a5,63
  15   │     srli a1,a1,63
  16   │     xori a1,a1,1
  17   │     and  a5,a5,a1
  18   │     srai a4,a0,63
  19   │     li   a3,-1
  20   │     srli a3,a3,1
  21   │     xor  a3,a3,a4
  22   │     neg  a4,a5
  23   │     and  a3,a3,a4
  24   │     addi a5,a5,-1
  25   │     and  a0,a2,a5
  26   │     or   a0,a0,a3
  27   │     ret

The below test suites are passed for this patch:
1. The rv64gcv fully regression test.

gcc/ChangeLog:

	* config/riscv/riscv-protos.h (riscv_expand_ssadd): Add new func
	decl for expanding ssadd.
	* config/riscv/riscv.cc (riscv_gen_sign_max_cst): Add new func
	impl to gen the max int rtx.
	(riscv_expand_ssadd): Add new func impl to expand the ssadd.
	* config/riscv/riscv.md (ssadd<mode>3): Add new pattern for
	signed integer .SAT_ADD.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/sat_arith.h: Add test helper macros.
	* gcc.target/riscv/sat_arith_data.h: Add test data.
	* gcc.target/riscv/sat_s_add-1.c: New test.
	* gcc.target/riscv/sat_s_add-2.c: New test.
	* gcc.target/riscv/sat_s_add-3.c: New test.
	* gcc.target/riscv/sat_s_add-4.c: New test.
	* gcc.target/riscv/sat_s_add-run-1.c: New test.
	* gcc.target/riscv/sat_s_add-run-2.c: New test.
	* gcc.target/riscv/sat_s_add-run-3.c: New test.
	* gcc.target/riscv/sat_s_add-run-4.c: New test.
	* gcc.target/riscv/scalar_sat_binary_run_xxx.h: New test.

Signed-off-by: Pan Li <pan2.li@intel.com>