RISC-V: Use helper function to get FPR to VR move cost

RISC-V: Use helper function to get FPR to VR move cost

Since last patch introduced get_fr2vr_cost () to get the correct cost to move
data from a floating-point to a vector register, this patch replaces existing
uses of the constant FR2VR.

gcc/ChangeLog:

	* config/riscv/riscv-vector-costs.cc (costs::adjust_stmt_cost): Replace
	FR2VR with get_fr2vr_cost ().
	* config/riscv/riscv.cc (riscv_register_move_cost): Likewise.
	(riscv_builtin_vectorization_cost): Likewise.