RISC-V: Add probability model of each block to prevent endless loop of Phase 3

RISC-V: Add probability model of each block to prevent endless loop of Phase 3

Notice that the PASS is just simpily pick the probability >= 50%
to do the backward fusion which will create endless loop on Phase 3.

Adding this probability to fix this bug.
gcc/ChangeLog:

	* config/riscv/riscv-vsetvl.cc
	(vector_infos_manager::vector_infos_manager): Add probability.
	(vector_infos_manager::dump): Ditto.
	(pass_vsetvl::compute_probabilities): Ditto.
	* config/riscv/riscv-vsetvl.h (struct vector_block_info): Ditto.