Allow trailing physreg RegisterSDNode operands on non-variadic instructions.

Also allow trailing register mask operands on non-variadic both
MachineSDNodes and MachineInstrs.

The extra physreg RegisterSDNode operands are added to the MI as
<imp-use> operands. This makes it possible to have non-variadic call
instructions.

Call and return instructions really are non-variadic, the argument
registers should only be used implicitly - they are not part of the
encoding.

llvm-svn: 159727