[X86] After turning VSELECT into SHRUNKBLEND, make we push the VSELECT into...

[X86] After turning VSELECT into SHRUNKBLEND, make we push the VSELECT into the worklist so it can be deleted.

Found while trying to figure out why my second version of D56421 worked better than the first version. We weren't deleting the vselect in a timely fashion and that caused SimplfyDemandedBit to see an additional user.

The new version doesn't have this problem so this fix isn't needed there, but seemed like the right thing to do.

llvm-svn: 350781