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

gcc/testsuite/ChangeLog:

	* g++.target/riscv/rvv/base/vsll_vx-1.C: New test.
	* g++.target/riscv/rvv/base/vsll_vx-2.C: New test.
	* g++.target/riscv/rvv/base/vsll_vx-3.C: New test.
	* g++.target/riscv/rvv/base/vsll_vx_mu-1.C: New test.
	* g++.target/riscv/rvv/base/vsll_vx_mu-2.C: New test.
	* g++.target/riscv/rvv/base/vsll_vx_mu-3.C: New test.
	* g++.target/riscv/rvv/base/vsll_vx_tu-1.C: New test.
	* g++.target/riscv/rvv/base/vsll_vx_tu-2.C: New test.
	* g++.target/riscv/rvv/base/vsll_vx_tu-3.C: New test.
	* g++.target/riscv/rvv/base/vsll_vx_tum-1.C: New test.
	* g++.target/riscv/rvv/base/vsll_vx_tum-2.C: New test.
	* g++.target/riscv/rvv/base/vsll_vx_tum-3.C: New test.
	* g++.target/riscv/rvv/base/vsll_vx_tumu-1.C: New test.
	* g++.target/riscv/rvv/base/vsll_vx_tumu-2.C: New test.
	* g++.target/riscv/rvv/base/vsll_vx_tumu-3.C: New test.