LoongArch: Fix ICE in highway-1.3.0 testsuite [PR121634]

I can't believe I made such a stupid pasto and the regression test
didn't detect anything wrong.

	PR target/121634

gcc/

	* config/loongarch/simd.md (simd_maddw_evod_<mode>_<su>): Use
	WVEC_HALF instead of WVEC for the mode of the sign_extend for
	the rhs of multiplication.

gcc/testsuite/

	* gcc.target/loongarch/pr121634.c: New test.

(cherry picked from commit 64932714a0786c6ba164cff3468f633cc67b23f5)