RISC-V: Add gr2vr cost helper function

RISC-V: Add gr2vr cost helper function

After we introduced the --param=gpr2vr-cost option to set the cost
value of when operation act from gpr to vr, we would like to introduce
a new helper function to get the cost of gp2vr.  And then make sure
all reference to gr2vr should go this helper function.

The helper function will pick up the GR2VR value if the above option is
not provided, or the default GR2VR will be returned.

gcc/ChangeLog:

	* config/riscv/riscv-protos.h (get_gr2vr_cost): Add new decl to
	get the cost of gr2vr.
	* config/riscv/riscv-vector-costs.cc (costs::adjust_stmt_cost):
	Leverage the helper function to get the cost of gr2vr.
	* config/riscv/riscv.cc (riscv_register_move_cost): Ditto.
	(riscv_builtin_vectorization_cost): Ditto.
	(get_gr2vr_cost): Add new impl of the helper function.

Signed-off-by: Pan Li <pan2.li@intel.com>