RISC-V: Add initial cost handling for segment loads/stores.

RISC-V: Add initial cost handling for segment loads/stores.

This patch makes segment loads and stores more expensive.  It adds
segment_permute_2 as well as 3 to 8 cost fields to the common vector
costs and adds handling to adjust_stmt_cost.

gcc/ChangeLog:

	* config/riscv/riscv-protos.h (struct common_vector_cost): Add
	segment_permute cost.
	* config/riscv/riscv-vector-costs.cc (costs::adjust_stmt_cost):
	Handle segment loads/stores.
	* config/riscv/riscv.cc: Initialize segment_permute_[2-8] to 1.

gcc/testsuite/ChangeLog:

	* gcc.dg/vect/costmodel/riscv/rvv/pr113112-4.c: Adjust test.