RISC-V: Support FP lrint/lrintf auto vectorization

This patch would like to support the FP lrint/lrintf auto vectorization.

* long lrint (double) for rv64
* long lrintf (float) for rv32

Due to the limitation that only the same size of data type are allowed
in the vectorier, the standard name lrintmn2 only act on DF => DI for
rv64, and SF => SI for rv32.

Given we have code like:

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

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

After this patch:
.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

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 (lrint<mode><vlconvert>2): New pattern
	for lrint/lintf.
	* config/riscv/riscv-protos.h (expand_vec_lrint): New func decl
	for expanding lint.
	* config/riscv/riscv-v.cc (emit_vec_cvt_x_f): New helper func impl
	for vfcvt.x.f.v.
	(expand_vec_lrint): New function impl for expanding lint.
	* config/riscv/vector-iterators.md: New mode attr and iterator.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/unop/test-math.h: New define for
	CVT like test case.
	* gcc.target/riscv/rvv/autovec/vls/def.h: Ditto.
	* gcc.target/riscv/rvv/autovec/unop/math-lrint-0.c: New test.
	* gcc.target/riscv/rvv/autovec/unop/math-lrint-1.c: New test.
	* gcc.target/riscv/rvv/autovec/unop/math-lrint-run-0.c: New test.
	* gcc.target/riscv/rvv/autovec/unop/math-lrint-run-1.c: New test.
	* gcc.target/riscv/rvv/autovec/vls/math-lrint-0.c: New test.
	* gcc.target/riscv/rvv/autovec/vls/math-lrint-1.c: New test.

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