Two types of instructions have register lists:

* LDM, et al, uses a bit mask to indicate the register list.
* VLDM, et al, uses a base register plus number.

The LDM instructions may be non-contiguous, but the VLDM ones must be
contiguous. Those are semantic checks that should be done later in the
compiler. Also postpone the creation of the bit mask until it's needed.

llvm-svn: 118640