RISC-V: Implement scalar SAT_TRUNC for signed integer

RISC-V: Implement scalar SAT_TRUNC for signed integer

This patch would like to implement the sstrunc for scalar signed
integer.

Form 1:
  #define DEF_SAT_S_TRUNC_FMT_1(WT, NT, NT_MIN, NT_MAX) \
  NT __attribute__((noinline))                          \
  sat_s_trunc_##WT##_to_##NT##_fmt_1 (WT x)             \
  {                                                     \
    NT trunc = (NT)x;                                   \
    return (WT)NT_MIN <= x && x <= (WT)NT_MAX           \
      ? trunc                                           \
      : x < 0 ? NT_MIN : NT_MAX;                        \
  }

DEF_SAT_S_TRUNC_FMT_1(int64_t, int32_t, INT32_MIN, INT32_MAX)

Before this patch:
  10   │ sat_s_trunc_int64_t_to_int32_t_fmt_1:
  11   │     li  a5,1
  12   │     slli    a5,a5,31
  13   │     li  a4,-1
  14   │     add a5,a0,a5
  15   │     srli    a4,a4,32
  16   │     bgtu    a5,a4,.L2
  17   │     sext.w  a0,a0
  18   │     ret
  19   │ .L2:
  20   │     srai    a5,a0,63
  21   │     li  a0,-2147483648
  22   │     xor a0,a0,a5
  23   │     not a0,a0
  24   │     ret

After this patch:
  10   │ sat_s_trunc_int64_t_to_int32_t_fmt_1:
  11   │     li  a5,-2147483648
  12   │     xori    a3,a5,-1
  13   │     slt a4,a0,a3
  14   │     slt a5,a5,a0
  15   │     and a5,a4,a5
  16   │     srai    a4,a0,63
  17   │     xor a4,a4,a3
  18   │     addi    a3,a5,-1
  19   │     neg a5,a5
  20   │     and a4,a4,a3
  21   │     and a0,a0,a5
  22   │     or  a0,a0,a4
  23   │     sext.w  a0,a0
  24   │     ret

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

gcc/ChangeLog:

	* config/riscv/riscv-protos.h (riscv_expand_sstrunc): Add new
	func decl to expand SAT_TRUNC.
	* config/riscv/riscv.cc (riscv_expand_sstrunc): Add new func
	impl to expand SAT_TRUNC.
	* config/riscv/riscv.md (sstrunc<mode><anyi_double_truncated>2):
	Add new pattern for double truncation.
	(sstrunc<mode><anyi_quad_truncated>2): Ditto but for quad.
	(sstrunc<mode><anyi_oct_truncated>2): Ditto but for oct.

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