[ARM] Diagnose ARM MOVT without :lower16: or :upper16: expression

This instruction was missing from the list of opcodes that we check, so we were
hitting an llvm_unreachable in ARMMCCodeEmitter.cpp for the ARM MOVT
instruction, rather than the diagnostic that is emitted for the other MOVW/MOVT
instructions.

Differential revision: https://reviews.llvm.org/D30936

llvm-svn: 297739