Fix merging base-updates for VLDM/VSTM: Before I switched these instructions

to use AddrMode4, there was a count of the registers stored in one of the
operands.  I changed that to just count the operands but forgot to adjust for
the size of D registers.  This was noticed by Evan as a performance problem
but it is a potential correctness bug as well, since it is possible that this
could merge a base update with a non-matching immediate.

llvm-svn: 113576