[x86] Replace some duplicated logic reasoning about whether particular
vector lanes can be modeled as zero with a call to the new function that
computes a bit-vector representing that information.

No functionality changed here, but will allow doing more clever things
with the zero-test.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218174 91177308-0d34-0410-b5e6-96231b3b80d8