RISC-V: Support partial VLS mode when preference fixed-vlmax [PR111857]

Given we have code like below:

typedef char vnx16i __attribute__ ((vector_size (16)));

vnx16i __attribute__ ((noinline, noclone))
test (vnx16i x, vnx16i y)
{
  return __builtin_shufflevector (x, y, 11, 12, 13, 14, 11, 12, 13, 14,
                                        11, 12, 13, 14, 11, 12, 13, 14);
}

It can perform the auto vectorization when

-march=rv64gcv_zvl1024b --param=riscv-autovec-preference=fixed-vlmax

but cannot when

-march=rv64gcv_zvl2048b --param=riscv-autovec-preference=fixed-vlmax

The reason comes from the miniaml machine mode of QI is RVVMF8QI, which
is 1024 / 8 = 128 bits, aka the size of VNx16QI. When we set zvl2048b,
the bit size of RVVMFQI is 2048 / 8 = 256, which is not matching the
bit size of VNx16QI (128 bits).

Thus, this patch would like to enable the VLS mode for such case, aka
VNx16QI vls mode for zvl2048b.

Before this patch:
test:
  srli    a4,a1,40
  andi    a4,a4,0xff
  srli    a3,a1,32
  srli    a5,a1,48
  slli    a0,a4,8
  andi    a3,a3,0xff
  andi    a5,a5,0xff
  slli    a2,a5,16
  or      a0,a3,a0
  srli    a1,a1,56
  or      a0,a0,a2
  slli    a2,a1,24
  slli    a3,a3,32
  or      a0,a0,a2
  slli    a4,a4,40
  or      a0,a0,a3
  slli    a5,a5,48
  or      a0,a0,a4
  or      a0,a0,a5
  slli    a1,a1,56
  or      a0,a0,a1
  mv      a1,a0
  ret

After this patch:
test:
  vsetivli        zero,16,e8,mf8,ta,ma
  vle8.v  v2,0(a1)
  vsetivli        zero,4,e32,mf2,ta,ma
  vrgather.vi     v1,v2,3
  vsetivli        zero,16,e8,mf8,ta,ma
  vse8.v  v1,0(a0)
  ret

	PR target/111857

gcc/ChangeLog:

	* config/riscv/riscv-opts.h (TARGET_VECTOR_VLS): Remove.
	* config/riscv/riscv-protos.h (vls_mode_valid_p): New func decl.
	* config/riscv/riscv-v.cc (autovectorize_vector_modes): Replace
	macro reference to func.
	(vls_mode_valid_p): New func impl for vls mode valid or not.
	* config/riscv/riscv-vector-switch.def (VLS_ENTRY): Replace
	macro reference to func.
	* config/riscv/vector-iterators.md: Ditto.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/vls-vlmax/perm-4.c: Adjust checker.
	* gcc.target/riscv/rvv/autovec/vls/def.h: Add help define.
	* gcc.target/riscv/rvv/autovec/vls-vlmax/pr111857-0.c: New test.
	* gcc.target/riscv/rvv/autovec/vls-vlmax/pr111857-1.c: New test.
	* gcc.target/riscv/rvv/autovec/vls-vlmax/pr111857-2.c: New test.
	* gcc.target/riscv/rvv/autovec/vls-vlmax/pr111857-3.c: New test.
	* gcc.target/riscv/rvv/autovec/vls-vlmax/pr111857-4.c: New test.
	* gcc.target/riscv/rvv/autovec/vls-vlmax/pr111857-5.c: New test.
	* gcc.target/riscv/rvv/autovec/vls-vlmax/pr111857-6.c: New test.

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