[X86] Rewrite vXi1 element insertion by using a vXi1 scalar_to_vector and...

[X86] Rewrite vXi1 element insertion by using a vXi1 scalar_to_vector and inserting into a vXi1 vector.

The existing code was already doing something very similar to subvector insertion so this allows us to remove the nearly duplicate code.

This patch is a little larger than it should be due to differences between the DQI handling between the two today.

llvm-svn: 323212