[FastISel][AArch64] Follow-up fix for "Fix shift-immediate emission for "zero" shifts."

Shifts also perform sign-/zero-extends to larger types, which requires us to emit
an integer extend instead of a simple COPY.

Related to PR21594.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222257 91177308-0d34-0410-b5e6-96231b3b80d8