ARM: partially handle 32-bit relocations for WoA

IMAGE_REL_ARM_MOV32T relocations require that the movw/movt pair-wise
relocation is not split up and reordered. When expanding the mov32imm
pseudo-instruction, create a bundle if the machine operand is referencing an
address.  This helps ensure that the relocatable address load is not reordered
by subsequent passes.

Unfortunately, this only partially handles the case as the Constant Island Pass
occurs after the instructions are unbundled and does not properly handle
bundles.  That is a more fundamental issue with the pass itself and beyond the
scope of this change.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207608 91177308-0d34-0410-b5e6-96231b3b80d8
ARM: move llvm_unreachable use

When building with -Werror=covered-switch-default (as on the buildbots), the
build would fail since all cases are covered by the switch.  Move the
llvm_unreachable to the end of the function as an annotation.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207609 91177308-0d34-0410-b5e6-96231b3b80d8