[x86] Fix a really silly bug that I introduced fixing another bug in the
new vector shuffle target DAG combines -- it helps to actually test for
the value you want rather than just using an integer in a boolean
context.

Have I mentioned that I loathe implicit conversions recently? :: sigh ::

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218576 91177308-0d34-0410-b5e6-96231b3b80d8