Fixed a few problems with vector shifts

  - when transforming a vector shift of a non-immediate scalar shift amount, zero
    extend the i32 shift amount to i64 since the vector shift reads 64 bits
  - when transforming i16 vectors to use a vector shift, zero extend i16 shift amount
  - improve the code quality in some cases when transforming vectors to use a vector shift

llvm-svn: 80935