[X86][AVX] Add test case showing incorrect extraction from VBROADCAST_LOAD on AVX2 targets

On AVX2 we tend to lower BUILD_VECTOR of constants as broadcasts if we can, in this case a <2 x i16> non-uniform constant has been lowered as a <4 x i32> broadcast.

The test case shows that the extraction folding code has incorrectly extracted the wrong part (lower WORD) of the resulting i32 memory source.

Found by internal fuzzing tests.