[X86][SSE] Added tests for insertion of zero elements into vectors

Many of these could be much better if we just lowered them all as shuffles - especially for the 256-bit vectors.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@256708 91177308-0d34-0410-b5e6-96231b3b80d8