RISC-V: Support FP lfloor/lfloorf auto vectorization

RISC-V: Support FP lfloor/lfloorf auto vectorization

This patch would like to support the FP lfloor/lfloorf auto vectorization.

* long lfloor (double) for rv64
* long lfloorf (float) for rv32

Due to the limitation that only the same size of data type are allowed
in the vectorier, the standard name lfloormn2 only act on DF => DI for
rv64, and SF => SI for rv32.

Given we have code like:

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

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

After this patch:
  frrm        a6
  ...
  fsrmi       2 // RDN
.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 (lfloor<mode><v_i_l_ll_convert>2): New
	pattern for lfloor/lfloorf.
	* config/riscv/riscv-protos.h (enum insn_type): New enum value.
	(expand_vec_lfloor): New func decl for expanding lfloor.
	* config/riscv/riscv-v.cc (expand_vec_lfloor): New func impl
	for expanding lfloor.

gcc/testsuite/ChangeLog:

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

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