[x86] Refactor the code for emitting INSERTPS to reuse the zeroable mask
analysis used elsewhere. This removes the last duplicate of this logic.
Also simplify the code here quite a bit. No functionality changed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218176 91177308-0d34-0410-b5e6-96231b3b80d8