Handle AddrMode4 for Thumb2 in rewriteT2FrameIndex. This occurs for

VLDM/VSTM instructions, and without this check, the code assumes that an
offset is allowed, as it would be with VLDR/VSTR.  The asm printer,
however, silently drops the offset, producing incorrect code.  Since the
address register in this case is either the stack or frame pointer, the
spill location ends up conflicting with some other stack slot or with
outgoing arguments on the stack.

llvm-svn: 81879