RISC-V: Add new option --param=gpr2vr-cost= for rvv insn

RISC-V: Add new option --param=gpr2vr-cost= for rvv insn

During investigate the combine from vec_dup and vop.vv into
vop.vx, we need to depend on the cost of the insn operate
from the gpr to vr, for example, vadd.vx.  Thus, for better
control and test, we introduce a new option, aka below:

--param=gpr2vr-cost=<unsigned int>

To specific the cost value of the insn that operate from
the gpr to vr.

gcc/ChangeLog:

	* config/riscv/riscv-opts.h (RVV_GR2VR_COST_UNPROVIDED): Add
	new macro to indicate the param is not provided.
	* config/riscv/riscv.opt: Add new option --pararm=gpr2vr-cost.

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