RISC-V: Support FP lround/lroundf auto vectorization

This patch would like to support the FP lround/lroundf auto vectorization.

* long lround (double) for rv64
* long lroundf (float) for rv32

Due to the limitation that only the same size of data type are allowed
in the vectorier, the standard name lroundmn2 only act on DF => DI for
rv64, and SF => SI for rv32.

Given we have code like:

void
test_lround (long *out, double *in, unsigned count)
{
  for (unsigned i = 0; i < count; i++)
    out[i] = __builtin_lround (in[i]);
}

Before this patch:
.L3:
  ...
  fld      fa5,0(a1)
  fcvt.l.d a5,fa5,rmm
  sd       a5,-8(a0)
  ...
  bne      a1,a4,.L3

After this patch:
  frrm     a6
  ...
  fsrmi    4 // RMM
.L3:
  ...
  vsetvli     a3,zero,e64,m1,ta,ma
  vfcvt.x.f.v v1,v1
  vsetvli     zero,a2,e64,m1,ta,ma
  vse32.v     v1,0(a0)
  ...
  bne         a2,zero,.L3
  ...
  fsrm     a6

The rest part like SF => DI/HF => DI/DF => SI/HF => SI will be covered
by TARGET_VECTORIZE_BUILTIN_VECTORIZED_FUNCTION.

gcc/ChangeLog:

	* config/riscv/autovec.md (lround<mode><v_i_l_ll_convert>2): New
	pattern for lround/lroundf.
	* config/riscv/riscv-protos.h (enum insn_type): New enum value.
	(expand_vec_lround): New func decl for expanding lround.
	* config/riscv/riscv-v.cc (expand_vec_lround): New func impl
	for expanding lround.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/unop/math-lround-0.c: New test.
	* gcc.target/riscv/rvv/autovec/unop/math-lround-1.c: New test.
	* gcc.target/riscv/rvv/autovec/unop/math-lround-run-0.c: New test.
	* gcc.target/riscv/rvv/autovec/unop/math-lround-run-1.c: New test.
	* gcc.target/riscv/rvv/autovec/vls/math-lround-0.c: New test.
	* gcc.target/riscv/rvv/autovec/vls/math-lround-1.c: New test.

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