[X86] Add test case for missed opportunity to recognize a vXi1 shuffle as an...

[X86] Add test case for missed opportunity to recognize a vXi1 shuffle as an insert into a zero vector.

We are currently missing this because shuffle canonicalization
puts the zero vector as V1 and the subvector as V2. Our current
code doesn't recognize this case.

llvm-svn: 369225