RISC-V: Add vsll.vx C++ API tests

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/base/vsll_vx-1.c: New test.
	* gcc.target/riscv/rvv/base/vsll_vx-2.c: New test.
	* gcc.target/riscv/rvv/base/vsll_vx-3.c: New test.
	* gcc.target/riscv/rvv/base/vsll_vx_m-1.c: New test.
	* gcc.target/riscv/rvv/base/vsll_vx_m-2.c: New test.
	* gcc.target/riscv/rvv/base/vsll_vx_m-3.c: New test.
	* gcc.target/riscv/rvv/base/vsll_vx_mu-1.c: New test.
	* gcc.target/riscv/rvv/base/vsll_vx_mu-2.c: New test.
	* gcc.target/riscv/rvv/base/vsll_vx_mu-3.c: New test.
	* gcc.target/riscv/rvv/base/vsll_vx_tu-1.c: New test.
	* gcc.target/riscv/rvv/base/vsll_vx_tu-2.c: New test.
	* gcc.target/riscv/rvv/base/vsll_vx_tu-3.c: New test.
	* gcc.target/riscv/rvv/base/vsll_vx_tum-1.c: New test.
	* gcc.target/riscv/rvv/base/vsll_vx_tum-2.c: New test.
	* gcc.target/riscv/rvv/base/vsll_vx_tum-3.c: New test.
	* gcc.target/riscv/rvv/base/vsll_vx_tumu-1.c: New test.
	* gcc.target/riscv/rvv/base/vsll_vx_tumu-2.c: New test.
	* gcc.target/riscv/rvv/base/vsll_vx_tumu-3.c: New test.