RISC-V: Use tu policy for first-element vec_set [PR115725].

This patch changes the tail policy for vmv.s.x from ta to tu.
By default the bug does not show up with qemu because qemu's
current vmv.s.x implementation always uses the tail-undisturbed
policy.  With a local qemu version that overwrites the tail
with ones when the tail-agnostic policy is specified, the bug
shows.

gcc/ChangeLog:

	* config/riscv/autovec.md: Add TU policy.
	* config/riscv/riscv-protos.h (enum insn_type): Define
	SCALAR_MOVE_MERGED_OP_TU.

gcc/testsuite/ChangeLog:

	PR target/115725

	* gcc.target/riscv/rvv/autovec/vls-vlmax/vec_set-1.c: Adjust
	test expectation.
	* gcc.target/riscv/rvv/autovec/vls-vlmax/vec_set-2.c: Ditto.
	* gcc.target/riscv/rvv/autovec/vls-vlmax/vec_set-3.c: Ditto.
	* gcc.target/riscv/rvv/autovec/vls-vlmax/vec_set-4.c: Ditto.