Fix the position of MemOperands in nodes that use variadic_ops

Fix the position of MemOperands in nodes that use variadic_ops
in DAGISelEmitter output. This bug was recently uncovered by the
addition of patterns for CALL32m and CALL64m, which are nodes
that now have both MemOperands and variadic_ops.

This bug was especially visible with PIC in various configurations,
because the new patterns are matching the indirect call code used
in many PIC configurations.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@51877 91177308-0d34-0410-b5e6-96231b3b80d8