[FastISel][AArch64] Add support for variable shift.

This adds the missing variable shift support for value type i8, i16, and i32.

This fixes <rdar://problem/18095685>.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216242 91177308-0d34-0410-b5e6-96231b3b80d8