Match: Support IMM=1 for unsigned scalar .SAT_SUB IMM form 1

This patch would like to support .SAT_SUB when one of the op
is IMM = 1 of form1.

Form 1:
 #define DEF_SAT_U_SUB_IMM_FMT_1(T, IMM) \
 T __attribute__((noinline))             \
 sat_u_sub_imm##IMM##_##T##_fmt_1 (T y)  \
 {                                       \
   return IMM >= y ? IMM - y : 0;        \
 }

Take below form 1 as example:
DEF_SAT_U_SUB_IMM_FMT_1(uint8_t, 1)

Before this patch:
__attribute__((noinline))
uint8_t sat_u_sub_imm1_uint8_t_fmt_1 (uint8_t y)
{
  uint8_t _1;
  uint8_t _3;

  <bb 2> [local count: 1073741824]:
  if (y_2(D) <= 1)
    goto <bb 3>; [41.00%]
  else
    goto <bb 4>; [59.00%]

  <bb 3> [local count: 440234144]:
  _3 = y_2(D) ^ 1;

  <bb 4> [local count: 1073741824]:
  # _1 = PHI <0(2), _3(3)>
  return _1;

}

After this patch:
__attribute__((noinline))
uint8_t sat_u_sub_imm1_uint8_t_fmt_1 (uint8_t y)
{
  uint8_t _1;

;;   basic block 2, loop depth 0
;;    pred:       ENTRY
  _1 = .SAT_SUB (1, y_2(D)); [tail call]
  return _1;
;;    succ:       EXIT

}

The below test suites are passed for this patch:
1. The rv64gcv fully regression tests.
2. The x86 bootstrap tests.
3. The x86 fully regression tests.

Signed-off-by: Li Xu <xuli1@eswincomputing.com>
gcc/ChangeLog:

	* match.pd: Support IMM=1.