Fix UNPREDICTABLE check in EmulateInstructionARM::EmulateADDRegShift

Summary:
As reported in LLVM bug 41487, the check in this function is wrong and should be
the same as the described check in the comment (which is correctly copied from the
ARM ISA reference).

Reviewers: #lldb, davide, JDevlieghere

Reviewed By: #lldb, davide, JDevlieghere

Subscribers: davide, javed.absar, kristof.beyls, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D60654

llvm-svn: 359387