Fix a bug with <8 x i16> shuffle lowering on X86 where parts of the

Fix a bug with <8 x i16> shuffle lowering on X86 where parts of the
shuffle could be skipped.  The check is invalid because the loop index i
doesn't correspond to the element actually inserted. The correct check is
already done a few lines earlier, for whether the element is already in
the right spot, so this shouldn't have any effect on the codegen for
code that was already correct.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@52486 91177308-0d34-0410-b5e6-96231b3b80d8