LoongArch: Add tests for ASX vector subtraction instructions.
gcc/testsuite/ChangeLog:
* gcc.target/loongarch/vector/lasx/lasx-xvssub-1.c: New test.
* gcc.target/loongarch/vector/lasx/lasx-xvssub-2.c: New test.
* gcc.target/loongarch/vector/lasx/lasx-xvsub.c: New test.
* gcc.target/loongarch/vector/lasx/lasx-xvsubi.c: New test.
* gcc.target/loongarch/vector/lasx/lasx-xvsubwev-1.c: New test.
* gcc.target/loongarch/vector/lasx/lasx-xvsubwev-2.c: New test.
* gcc.target/loongarch/vector/lasx/lasx-xvsubwod-1.c: New test.
* gcc.target/loongarch/vector/lasx/lasx-xvsubwod-2.c: New test.