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

This change emits a COPY for a shift-immediate with a "zero" shift value.
This fixes PR21594 where we emitted a shift instruction with an incorrect
immediate operand.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222247 91177308-0d34-0410-b5e6-96231b3b80d8