Partially revert "LoongArch: Fix bug123807."

This reverts the loongarch.cc change of the commit
4df77a254263e96af1ab4d1288a35cff10c515a4.

PR 123807 turns out to be a special case of the middle-end PR 124250.
The previous ad-hoc fix is unneeded now since the underlying middle-end
issue is fixed, so revert it but keep the test case.

gcc/

	PR target/123807
	PR middle-end/124250
	* config/loongarch/loongarch.cc
	(loongarch_expand_vector_init_same): Revert r16-7163 change.