RISC-V: Change vsetvl tail and mask policy to default policy

RISC-V: Change vsetvl tail and mask policy to default policy

This patch change the vsetvl policy to default policy
(returned by get_prefer_mask_policy and get_prefer_tail_policy) instead
fixed policy. Any policy is now returned, allowing change to agnostic
or undisturbed. In the future, users may be able to control the default
policy, such as keeping agnostic by compiler options.

gcc/ChangeLog:

	* config/riscv/riscv-protos.h (IS_AGNOSTIC): Move to here.
	* config/riscv/riscv-v.cc (gen_no_side_effects_vsetvl_rtx):
	Change to default policy.
	* config/riscv/riscv-vector-builtins-bases.cc: Change to default policy.
	* config/riscv/riscv-vsetvl.h (IS_AGNOSTIC): Delete.
	* config/riscv/riscv.cc (riscv_print_operand): Use IS_AGNOSTIC to test.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/base/binop_vx_constraint-171.c: Adjust.
	* gcc.target/riscv/rvv/base/binop_vx_constraint-173.c: Adjust.
	* gcc.target/riscv/rvv/vsetvl/vsetvl-24.c: New test.