LoongArch: Implement vector cbranch optab for LSX and LASX

In order to support vectorization of loops with multiple exits, this
patch adds the implementation of the conditional branch optab for
LoongArch LSX/LASX instructions.

This patch causes the gen-vect-{2,25}.c tests to fail.  This is because
the support for vectorizing loops with multiple exits has vectorized
the loop checking the results.  The failure is due to an issue in the
test case's own implementation.

gcc/ChangeLog:

	* config/loongarch/simd.md (cbranch<mode>4): New expander.

gcc/testsuite/ChangeLog:

	* lib/target-supports.exp (check_effective_target_vect_early_break_hw,
	check_effective_target_vect_early_break): Support LoongArch LSX.
	* gcc.target/loongarch/vector/lasx/lasx-vseteqz.c: New test.
	* gcc.target/loongarch/vector/lsx/lsx-vseteqz.c: New test.

Co-authored-by: Deng Jianbo <dengjianbo@loongson.cn>