VECT: Support LEN_MASK_{LOAD,STORE} ifn && optabs

This patch adds LEN_MASK_ LOAD/STORE to support flow control for targets
like RISC-V that uses length in loop control.
Normalize load/store into LEN_MASK_ LOAD/STORE as long as either length
or mask is valid. Length is the outcome of SELECT_VL or MIN_EXPR.
Mask is the outcome of comparison.

LEN_MASK_ LOAD/STORE format is defined as follows:
1). LEN_MASK_LOAD (ptr, align, length, mask).
2). LEN_MASK_STORE (ptr, align, length, mask, vec).

Consider these 4 following cases:

VLA: Variable-length auto-vectorization
VLS: Specific-length auto-vectorization

Case 1 (VLS): -mrvv-vector-bits=128   IR (Does not use LEN_MASK_*):
Code:					v1 = MEM (...)
  for (int i = 0; i < 4; i++)           v2 = MEM (...)
    a[i] = b[i] + c[i];                 v3 = v1 + v2
                                        MEM[...] = v3

Case 2 (VLS): -mrvv-vector-bits=128   IR (LEN_MASK_* with length = VF, mask = comparison):
Code:                                   mask = comparison
  for (int i = 0; i < 4; i++)           v1 = LEN_MASK_LOAD (length = VF, mask)
    if (cond[i])                        v2 = LEN_MASK_LOAD (length = VF, mask)
      a[i] = b[i] + c[i];               v3 = v1 + v2
                                        LEN_MASK_STORE (length = VF, mask, v3)

Case 3 (VLA):
Code:                                   loop_len = SELECT_VL or MIN
  for (int i = 0; i < n; i++)           v1 = LEN_MASK_LOAD (length = loop_len, mask = {-1,-1,...})
      a[i] = b[i] + c[i];               v2 = LEN_MASK_LOAD (length = loop_len, mask = {-1,-1,...})
                                        v3 = v1 + v2
                                        LEN_MASK_STORE (length = loop_len, mask = {-1,-1,...}, v3)

Case 4 (VLA):
Code:                                   loop_len = SELECT_VL or MIN
  for (int i = 0; i < n; i++)           mask = comparison
      if (cond[i])                      v1 = LEN_MASK_LOAD (length = loop_len, mask)
      a[i] = b[i] + c[i];               v2 = LEN_MASK_LOAD (length = loop_len, mask)
                                        v3 = v1 + v2
                                        LEN_MASK_STORE (length = loop_len, mask, v3)

Co-authored-by: Robin Dapp <rdapp.gcc@gmail.com>

gcc/ChangeLog:

	* doc/md.texi: Add len_mask{load,store}.
	* genopinit.cc (main): Ditto.
	(CMP_NAME): Ditto.
	* internal-fn.cc (len_maskload_direct): Ditto.
	(len_maskstore_direct): Ditto.
	(expand_call_mem_ref): Ditto.
	(expand_partial_load_optab_fn): Ditto.
	(expand_len_maskload_optab_fn): Ditto.
	(expand_partial_store_optab_fn): Ditto.
	(expand_len_maskstore_optab_fn): Ditto.
	(direct_len_maskload_optab_supported_p): Ditto.
	(direct_len_maskstore_optab_supported_p): Ditto.
	* internal-fn.def (LEN_MASK_LOAD): Ditto.
	(LEN_MASK_STORE): Ditto.
	* optabs.def (OPTAB_CD): Ditto.

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