[InstCombine][SSE4a] Fix assertion failure in the insertq/insertqi combining logic.

This fixes a similar issue to the one already fixed by r280804
(revieved in D24256). Revision 280804 fixed the problem with unsafe dyn_casts
in the extrq/extrqi combining logic. However, it turns out that even the
insertq/insertqi logic was affected by the same problem.

llvm-svn: 280807