[x86] Remove the duplicated code for testing whether we can widen the
elements of a shuffle mask and simplify how it works. No functionality
changed now that the bug that was here has been fixed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215696 91177308-0d34-0410-b5e6-96231b3b80d8