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.

llvm-svn: 52486