RISC-V: Add the missed combine of [u]int64 -> _Float16 and vcond

RISC-V: Add the missed combine of [u]int64 -> _Float16 and vcond

Hi,

This patch let the INT64 to FP16 convert split to two small converts
(INT64 -> FP32 and FP32 -> FP16) when expanding instead of dealy the
split to split1 pass. This change could make it possible to combine
the FP32 to FP16 and vcond patterns and so we don't need to add an
combine pattern for INT64 to FP16 and vcond patterns.

Consider this code:
  void
  foo (_Float16 *__restrict r, int64_t *__restrict a, _FLoat16 *__restrict b,
       int64_t *__restrict pred, int n)
  {
    for (int i = 0; i < n; i += 1)
      {
        r[i] = pred[i] ? (_Float16) a[i] : b[i];
      }
  }

Before this patch:
  ...
  vfncvt.f.f.w    v2,v2
  vmerge.vvm      v1,v1,v2,v0
  vse16.v v1,0(a0)
  ...

After this patch:
  ...
  vfncvt.f.f.w    v1,v2,v0.t
  vse16.v v1,0(a0)
  ...

gcc/ChangeLog:

	* config/riscv/autovec.md (<float_cvt><mode><vnnconvert>2):
	Change to define_expand.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/cond/cond_convert_int2float-rv32-1.c:
	Add vfncvt.f.f.w assert.
	* gcc.target/riscv/rvv/autovec/cond/cond_convert_int2float-rv32-2.c:
	Ditto.
	* gcc.target/riscv/rvv/autovec/cond/cond_convert_int2float-rv64-1.c:
	Ditto.
	* gcc.target/riscv/rvv/autovec/cond/cond_convert_int2float-rv64-2.c:
	Ditto.