On AVX, we can load v8i32 at a time. The bug happens when two uneven loads are used.

When we load the v12i32 type, the GenWidenVectorLoads method generates two loads: v8i32 and v4i32 
and attempts to use CONCAT_VECTORS to join them. In this fix I concat undef values to widen 
the smaller value. The test "widen_load-2.ll" also exposes this bug on AVX.

llvm-svn: 147964