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

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@113576 91177308-0d34-0410-b5e6-96231b3b80d8