[X86] Use correct subvector index when combining two insert subvectors featuring...

[X86] Use correct subvector index when combining two insert subvectors featuring zero vectors.

Previously we were using one of the subvector indices twice. The included test case causes an assert without this change.

Thanks to Simon Pilgrim for catching this.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@314429 91177308-0d34-0410-b5e6-96231b3b80d8