RISC-V: Optimize combine sequence by merge approach

gcc/ChangeLog:

	* config/riscv/riscv-v.cc
	(rvv_builder::combine_sequence_use_merge_profitable_p): New function.
	(expand_vector_init_merge_combine_sequence): Ditto.
	(expand_vec_init): Adapt for new optimization.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/vls-vlmax/combine-merge-1.c: New test.
	* gcc.target/riscv/rvv/autovec/vls-vlmax/combine-merge-2.c: New test.
	* gcc.target/riscv/rvv/autovec/vls-vlmax/combine-merge_run-1.c: New test.
	* gcc.target/riscv/rvv/autovec/vls-vlmax/combine-merge_run-2.c: New test.
	* gcc.target/riscv/rvv/autovec/vls/combine-merge-1.c: New test.
	* gcc.target/riscv/rvv/autovec/vls/combine-merge-10.c: New test.
	* gcc.target/riscv/rvv/autovec/vls/combine-merge-11.c: New test.
	* gcc.target/riscv/rvv/autovec/vls/combine-merge-12.c: New test.
	* gcc.target/riscv/rvv/autovec/vls/combine-merge-13.c: New test.
	* gcc.target/riscv/rvv/autovec/vls/combine-merge-14.c: New test.
	* gcc.target/riscv/rvv/autovec/vls/combine-merge-2.c: New test.
	* gcc.target/riscv/rvv/autovec/vls/combine-merge-3.c: New test.
	* gcc.target/riscv/rvv/autovec/vls/combine-merge-4.c: New test.
	* gcc.target/riscv/rvv/autovec/vls/combine-merge-5.c: New test.
	* gcc.target/riscv/rvv/autovec/vls/combine-merge-6.c: New test.
	* gcc.target/riscv/rvv/autovec/vls/combine-merge-7.c: New test.
	* gcc.target/riscv/rvv/autovec/vls/combine-merge-8.c: New test.
	* gcc.target/riscv/rvv/autovec/vls/combine-merge-9.c: New test.