LoongArch: Improve lasx_xvpermi_q_<LASX:mode> insn pattern

For instruction xvpermi.q, unused bits in operands[3] need be set to 0 to avoid
causing undefined behavior on LA464.

gcc/ChangeLog:

	* config/loongarch/lasx.md: Set the unused bits in operand[3] to 0.

gcc/testsuite/ChangeLog:

	* gcc.target/loongarch/vector/lasx/lasx-xvpremi.c: Removed.
	* gcc.target/loongarch/vector/lasx/lasx-xvpermi_q.c: New test.