Vect: Support IFN SAT_TRUNC for unsigned vector int

This patch would like to support the .SAT_TRUNC for the unsigned
vector int.  Given we have below example code:

Form 1
  #define VEC_DEF_SAT_U_TRUC_FMT_1(NT, WT)                             \
  void __attribute__((noinline))                                       \
  vec_sat_u_truc_##WT##_to_##NT##_fmt_1 (NT *x, WT *y, unsigned limit) \
  {                                                                    \
    for (unsigned i = 0; i < limit; i++)                               \
      {                                                                \
        bool overflow = y[i] > (WT)(NT)(-1);                           \
        x[i] = ((NT)y[i]) | (NT)-overflow;                             \
      }                                                                \
  }

VEC_DEF_SAT_U_TRUC_FMT_1 (uint32_t, uint64_t)

Before this patch:
void vec_sat_u_truc_uint64_t_to_uint32_t_fmt_1 (uint32_t * x, uint64_t * y, unsigned int limit)
{
  ...
  _51 = .SELECT_VL (ivtmp_49, POLY_INT_CST [2, 2]);
  ivtmp_35 = _51 * 8;
  vect__4.7_32 = .MASK_LEN_LOAD (vectp_y.5_34, 64B, { -1, ... }, _51, 0);
  mask_overflow_16.8_30 = vect__4.7_32 > { 4294967295, ... };
  vect__5.9_29 = (vector([2,2]) unsigned int) vect__4.7_32;
  vect__10.13_20 = .VCOND_MASK (mask_overflow_16.8_30, { 4294967295, ... }, vect__5.9_29);
  ivtmp_12 = _51 * 4;
  .MASK_LEN_STORE (vectp_x.14_11, 32B, { -1, ... }, _51, 0, vect__10.13_20);
  vectp_y.5_33 = vectp_y.5_34 + ivtmp_35;
  vectp_x.14_46 = vectp_x.14_11 + ivtmp_12;
  ivtmp_50 = ivtmp_49 - _51;
  if (ivtmp_50 != 0)
  ...
}

After this patch:
void vec_sat_u_truc_uint64_t_to_uint32_t_fmt_1 (uint32_t * x, uint64_t * y, unsigned int limit)
{
  ...
  _12 = .SELECT_VL (ivtmp_21, POLY_INT_CST [2, 2]);
  ivtmp_34 = _12 * 8;
  vect__4.7_31 = .MASK_LEN_LOAD (vectp_y.5_33, 64B, { -1, ... }, _12, 0);
  vect_patt_40.8_30 = .SAT_TRUNC (vect__4.7_31); // << .SAT_TRUNC
  ivtmp_29 = _12 * 4;
  .MASK_LEN_STORE (vectp_x.9_28, 32B, { -1, ... }, _12, 0, vect_patt_40.8_30);
  vectp_y.5_32 = vectp_y.5_33 + ivtmp_34;
  vectp_x.9_27 = vectp_x.9_28 + ivtmp_29;
  ivtmp_20 = ivtmp_21 - _12;
  if (ivtmp_20 != 0)
  ...
}

The below test suites are passed for this patch
* The x86 bootstrap test.
* The x86 fully regression test.
* The rv64gcv fully regression tests.

gcc/ChangeLog:

	* tree-vect-patterns.cc (gimple_unsigned_integer_sat_trunc): Add
	new decl generated by match.
	(vect_recog_sat_trunc_pattern): Add new func impl to recog the
	.SAT_TRUNC pattern.

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