VECT: Support SLP MASK_LEN_GATHER_LOAD with conditional mask

VECT: Support SLP MASK_LEN_GATHER_LOAD with conditional mask

This patch leverage current MASK_GATHER_LOAD to support SLP MASK_LEN_GATHER_LOAD with condtional mask.

Unconditional MASK_LEN_GATHER_LOAD (base, offset, scale, zero, -1) SLP is not included in this patch
since it seems that we can't support it in the middle-end:

FAIL: gcc.dg/tree-ssa/pr44306.c (internal compiler error: in vectorizable_load, at tree-vect-stmts.cc:9885)

May be we should support GATHER_LOAD explictily in RISC-V backend to walk around this issue.

I am gonna support GATHER_LOAD explictly work around in RISC-V backend.

This patch also adds conditional gather load test since there is no conditional gather load test.

Ok for trunk ?

gcc/ChangeLog:

	* tree-vect-slp.cc (vect_get_operand_map): Add MASK_LEN_GATHER_LOAD.
	(vect_build_slp_tree_1): Ditto.
	(vect_build_slp_tree_2): Ditto.

gcc/testsuite/ChangeLog:

	* gcc.dg/vect/vect-gather-6.c: New test.