Fix PR18054

- Fix bug in (vsext (vzext x)) -> (vsext x) in SIGN_EXTEND_IN_REG
  lowering where we need to check whether x is a vector type (in-reg
  type) of i8, i16 or i32; otherwise, that optimization is not valid.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@195779 91177308-0d34-0410-b5e6-96231b3b80d8