[X86][AVX2] Fix issue in lowerV8I16GeneralSingleInputVectorShuffle that was assuming v8i16 vectors

We can use this with v16i16/v32i16 as well.

Found during fuzz testing.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@305472 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed