[X86] If we see an insert of a bitcast into zero vector, canonicalize it to move...

[X86] If we see an insert of a bitcast into zero vector, canonicalize it to move the bitcast to the other side of the insert.

This improves detection of zeroing of upper bits during isel.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@315161 91177308-0d34-0410-b5e6-96231b3b80d8